International Journal of Gynecology and Obstetrics 127 (2014) 248–253
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CLINICAL ARTICLE
Trends in preterm birth in singleton deliveries in a Hong Kong population Annie S.Y. Hui a,⁎, Terence T. Lao a, Tak Yeung Leung a, Jelle M. Schaaf b, Daljit S. Sahota a a b
Department of Obstetrics and Gynaecology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong Special Administrative Region Department of Medical Informatics, Academic Medical Centre, Amsterdam, Netherlands
a r t i c l e
i n f o
Article history: Received 22 January 2014 Received in revised form 23 June 2014 Accepted 1 August 2014 Keywords: Perinatal mortality Preterm birth Preterm premature rupture of membranes
a b s t r a c t Objective: To examine trends in preterm birth and its relationship with perinatal mortality in Hong Kong. Methods: In a retrospective cohort study, data were reviewed from singletons delivered between 1995 and 2011 at a university teaching hospital. Trends in preterm birth (between 24 and 36 weeks of pregnancy), perinatal mortality, and subtypes of preterm birth (spontaneous, iatrogenic, and following preterm premature rupture of membranes [PPROM]) were examined via linear regression. Results: There were 103 364 singleton deliveries, of which 6722 (6.5%) occurred preterm, including 1835 (1.8%) early preterm births (24–33 weeks) and 4887 (4.7%) late preterm births (34–36 weeks). Frequency of preterm birth remained fairly consistent over the study period, but that of spontaneous preterm birth decreased by 25% (β = –0.83; P b 0.001), from 4.5% to 3.8%. Frequency of preterm birth following PPROM increased by 135% (β = 0.82; P b 0.001), from 0.7% to 1.7%. The perinatal mortality rate decreased from 56.7 to 37.0 deaths per 1000 deliveries before 37 weeks (β = –0.16; P = 0.54). Early preterm birth contributed to 16.0% of all deaths. Conclusion: Although the overall rate of preterm birth in Hong Kong has remained constant, the frequencies of its subtypes have changed. Overall perinatal mortality is gradually decreasing, but early preterm birth remains a major contributor. © 2014 International Federation of Gynecology and Obstetrics. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Despite the increased accessibility and improved standards of prenatal care in most countries, the incidence of preterm birth has continued to rise over the past two decades in some nations [1]. Late preterm birth (at 34–36 weeks of gestation) is still the most common type, accounting for up to 72% of all preterm births in the USA in 2012 [2]. Although survival of preterm newborns has greatly improved because of advances in neonatal intensive care, the social and economic burdens of preterm birth remain substantial. Preterm-related complications are estimated to account for more than 35% of the 3.1 million neonatal deaths that occur worldwide every year, and preterm birth is still the second most common cause of death among children younger than 5 years [3]. In Asia, the estimated incidence of preterm birth varies from 7.2% in East Asia to 13.6% in Southeast Asia [1]. The reason for the differences among Asian regions is unclear, but a comparison between Chinese women living in China, Hong Kong, and Australia showed that the frequency of preterm birth was higher for those residing in high-income regions [4]. This variation might be partly related to increased medical interventions. In the USA, for example, an increase in the frequency of preterm birth was caused mainly by a rise in the number of medically
⁎ Corresponding author at: 1E, Department of Obstetrics and Gynecology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong Special Administrative Region. Tel.: +852 26322211; fax: +852 26360008. E-mail address: [email protected] (A.S.Y. Hui).
indicated deliveries, whereas spontaneous preterm birth decreased [5,6]; once healthcare providers became aware of the problem, efforts were successfully made to reduce the preterm rates from 11.1% in 2006 to 9.9% in 2012 [2]. Nevertheless, the overall frequency of preterm birth provides no information on the role of medical interventions or on the relative contributions of spontaneous versus iatrogenic preterm birth to the perinatal mortality rate among preterm neonates. Hong Kong lies between low-resource and high-resource countries in terms of economic development. In 2011, 93.6% of the 7.07 million individuals in Hong Kong were of Chinese ethnic origin [7]. Therefore, studies in Hong Kong of the trends in preterm birth and their impact on perinatal mortality would not be confounded by variation in ethnic origin due to population migration and immigration, and would help to elucidate the influence of changing obstetric practice and the associated perinatal mortality so that appropriate strategies for preventing preterm birth and reducing perinatal mortality can be devised. It is also informative to examine the trends in early and late preterm birth, and in the three clinical subtypes (spontaneous preterm birth, iatrogenic preterm birth, and preterm births following preterm premature rupture of membranes [PPROM]). It is helpful to consider preterm birth following PPROM as a stand-alone subtype because—according to the latest Royal College of Obstetricians and Gynaecologists guideline for PPROM [8]—induction after PPROM is recommended at or after 34 weeks if there is no spontaneous onset of labor. Thus, preterm birth following PPROM includes both spontaneous and iatrogenic births. The primary aim of the present study was to examine trends in preterm birth and associations with perinatal mortality among singleton
http://dx.doi.org/10.1016/j.ijgo.2014.06.019 0020-7292/© 2014 International Federation of Gynecology and Obstetrics. Published by Elsevier Ireland Ltd. All rights reserved.
A.S.Y. Hui et al. / International Journal of Gynecology and Obstetrics 127 (2014) 248–253 Table 1 Frequency of preterm birth among 103 364 singleton deliveries. Type
No. (%)
Preterm birtha Early preterm birthb Late preterm birthc Spontaneous preterm birth Spontaneous preterm birth after PPROM Iatrogenic preterm birth Iatrogenic preterm birth after PPROM
6722 (6.5) 1835 (1.8) 4887 (4.7) 4266 (4.1) 338 (0.3) 2456 (2.4) 754 (0.7)
Abbreviation: PPROM, premature rupture of membrane. a Between 24 and 36 weeks, 6 days. b Between 24 and 33 weeks, 6 days. c Between 34 and 36 weeks, 6 days.
deliveries in Hong Kong. A secondary aim was to carry out a subgroup analysis for iatrogenic preterm birth to examine common indications and their associated perinatal mortality risk.
2. Materials and methods In a retrospective cohort study, data were reviewed from all singletons delivered between January 1, 1995, and October 31, 2011, at the Prince of Wales Hospital in Hong Kong—a university obstetrics unit taking care of both high- and low-risk women, with more than 6000 deliveries per year. Ethics approval was obtained from the Joint Chinese University of Hong Kong–New Territories East Cluster Clinical Research Ethics Committee (reference number 2012.256). Informed consent was not required because the data did not contain any identifying information. The study data were obtained from the Obstetric Specialty Clinical Information System database, which was set up by the Hong Kong Hospital Authority to record all maternal and perinatal outcomes in public hospitals. For this database, prenatal information and perinatal outcomes are prospectively recorded at each visit and at the time of delivery by the attending midwife. The accuracy and validity of the data are checked by a second midwife. Moreover, perinatal outcomes are reviewed in monthly and annual audit meetings to ensure that all adverse events were identified. Gestational age was estimated on the basis of the date of last menstrual period and/or crown–rump length measurement during early dating ultrasound. A service for anomaly scans in the second trimester was introduced in 1997, and combined Down syndrome screening in the first trimester in 2003. Therefore, most women who delivered in the study period had had at least one prenatal scan. These scans were
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used to revise the estimated delivery date and gestational age if they were inconsistent with those calculated from the last menstrual period. Preterm birth was defined as delivery between the start of week 24 and the end of week 36. Early preterm birth occurred between the start of week 24 and the end of week 33, and late preterm birth occurred between the start of week 34 and the end of week 36. Pregnancies that ended before week 24, including spontaneous abortions and medically or surgically terminated pregnancies, were excluded. Preterm birth was classified into three subtypes: spontaneous preterm birth (birth following spontaneous onset of labor), iatrogenic preterm birth (medically indicated deliveries, with either induction of labor or cesarean), and preterm birth following PPROM (including both spontaneous and iatrogenic births). Perinatal death was defined according to the US National Centre of Health Statistics, American Academy of Pediatrics Committee on Fetus and Newborn, and the American College of Obstetricians and Gynecologists Committee on Obstetric Practice [9,10]. The minimum gestational age was adjusted according to the legal definition of viability in Hong Kong (24 weeks’ gestation). Therefore, in the present study, the overall perinatal mortality rate was defined as the sum of fetal deaths (≥ 24 weeks of gestation) plus neonatal deaths within the first 28 days of delivery, expressed per 1000 deliveries. Specific mortality risks were calculated per 1000 deliveries of the specified gestation or clinical condition. The absolute and relative trends in the incidence of preterm birth overall and in the three subtypes, and in overall perinatal mortality were examined. Intrauterine deaths and lethal fetal anomalies were common confounders, because induction of labor was typically offered before term in these cases in the study cohort, and so they would be counted as iatrogenic preterm births in most cases. Therefore, in addition to calculating overall perinatal mortality, the analysis was repeated without these cases, thereby generating a corrected perinatal mortality rate to reflect more accurately the contribution of preterm birth to perinatal mortality. Perinatal mortality for iatrogenic preterm birth was also examined. Common indications were categorized and the specific mortality risk was calculated. Statistical analysis was performed via PASW Statistics for Windows version 18.0 (IBM, Armonk, NY, USA). Descriptive analysis was used to calculate the prevalence of preterm birth. Trend analysis was undertaken by performing a linear-by-linear trend test, and then by fitting a linear regression model for each series and testing the regression coefficient (β) to see whether there was significant deviation from zero via a t test. For comparisons between groups, χ2 tests were used as appropriate. All statistical tests were two-tailed. P b 0.05 was considered significant.
Table 2 Preterm births and related mortality.a Gestational age at delivery
24–33 weeks Total Iatrogenic Spontaneous 34–36 weeks Total Iatrogenic Spontaneous ≥37 weeks Total Iatrogenic Spontaneous a b c d
Deliveries (n = 103 364)
Perinatal deaths (n = 493)b
Overall perinatal mortality rate (per 1000 deliveries)
Intrauterine deaths and lethal anomalies (n = 348)c
1835 (1.8) 698 (0.7) 1137 (1.1)
268 (54.4) 178 (36.1) 90 (18.3)
146.0 255.0 79.2
189 (54.3) 156 (44.8) 33 (9.5)
4887 (4.7) 1758 (1.7) 3129 (3.0)
65 (13.2) 45 (9.1) 20 (4.1)
13.3 25.6 6.4
96 642 (93.5) 23 999 (23.2) 72 643 (70.3)
160 (32.5) 89 (18.1) 71 (14.4)
1.7 3.7 1.0
Corrected number of deliveries (n = 103 016)
Corrected number of perinatal deaths (n = 145)
Corrected perinatal mortality rate (per 1000 deliveries)
Corrected contribution to overall mortality (%)d
1646 (1.6) 542 (0.5) 1104 (1.1)
79 (54.5) 22 (15.2) 57 (39.3)
48.0 40.6 51.6
16.0 4.5 11.6
48 (13.8) 36 (10.3) 12 (3.4)
4839 (4.7) 1722 (1.7) 3117 (3.0)
17 (11.7) 9 (6.2) 8 (5.5)
3.5 5.2 2.6
3.4 1.8 1.6
111 (31.9) 66 (19.0) 45 (12.9)
96 531 (93.7) 23 933 (23.2) 72 598 (70.5)
49 (33.8) 23 (15.9) 26 (17.9)
0.5 1.0 0.4
9.9 4.7 5.3
Values are given as number (percentage) unless otherwise indicated. Fetal deaths (≥24 gestational weeks) plus neonatal deaths within the first 28 days of birth. Intrauterine deaths were intrauterine fetal deaths at a gestational age of less than 24 weeks. Percentages calculated with total number of perinatal deaths to reflect overall contribution to mortality.
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3. Results
Table 3 Causes of iatrogenic preterm birth and associated perinatal mortality risks.a Cause
Gestational age, wk
Iatrogenic Perinatal preterm deaths births (n = 223) (n = 2456)
PPROM Clinical chorioamnionitis Hypertensive disorders in pregnancy Severe pre-eclampsia Eclampsia Placental abruption Fetal compromise Fetal compromise (primary) Intrauterine growth restriction Fetal distress Suboptimal CTG Oligohydramnios Cord incidents Prepartum hemorrhage Placenta previa Placental abruption Not specified Maternal diseases Maternal compromise Fetal complications Poor obstetrics history Intrauterine death Fetal anomaly
34.98 ± 1.49 754 (30.7) 32.35 ± 3.20 72 (2.9) 33.19 ± 2.78 548 (22.3)
6 (2.7) 6 (2.7) 7 (3.1)
8.0 83.3 12.9
33.43 32.71 31.50 32.09 34.35 33.97
± ± ± ± ± ±
2.74 446 (18.2) 3.45 7 (0.3) 1.87 6 (0.2) 2.70 89 (3.6) 2.25 407 (16.6) 2.31 154 (6.3)
5 (2.2) 0 0 2 (0.9) 8 (3.6) 2 (0.9)
11.2
33.64 35.15 35.67 33.89 34.16 34.23 32.91 34.53 34.62 34.57 33.57 36.00 29.70 32.56
± ± ± ± ± ± ± ± ± ± ± ± ± ±
2.62 1.27 0.67 3.16 2.27 2.01 2.92 2.62 2.12 2.18 1.81 0.00 3.76 2.85
4 (1.8) 1 (0.4) 1 (0.4) 0 3 (1.3) 2 (0.9) 1 (0.4) 0 1 (0.4) 1 (0.4) 0 0 156 (70.0) 36 (16.1)
107 (4.4) 67 (2.7) 64 (2.6) 15 (0.6) 353 (14.4) 249 (10.1) 34 (1.4) 70 (2.9) 89 (3.6) 74 (3.0) 7 (0.3) 8 (0.3) 156 (6.4) 77 (3.1)
Perinatal mortality (per 1000 deliveries)
22.5 19.7 13.0 37.4 14.9 15.6 8.5 8.0 29.4 11.2 13.5
1000 467.5
Abbreviation: PPROM, preterm premature rupture of membranes. a Values are given as mean ± SD or number (percentage), unless otherwise indicated.
Over the 17-year study period, there were 103 364 singleton deliveries, among which 99 457 (96.2%) mothers were of Chinese ethnic origin. Overall, 6722 (6.5%) births occurred preterm, including 1835 (1.8%) early preterm births and 4887 (4.7%) late preterm births (Table 1). Spontaneous preterm birth was recorded for 4266 (4.1%) deliveries, iatrogenic preterm birth for 2456 (2.4%), and preterm birth after PPROM for 1092 (1.1%). A total of 493 perinatal deaths occurred, giving an overall perinatal mortality rate of 4.77 deaths per 1000 deliveries. Table 2 shows the contribution of preterm birth to perinatal mortality. After excluding 348 intrauterine deaths and lethal fetal anomalies, the corrected perinatal mortality rate was 1.4 per 1000 deliveries. Of the 145 perinatal deaths that were not intrauterine or related to lethal anomalies, 79 (54.5%) occurred after early preterm birth and 17 (11.7%) after late preterm birth. The mortality rate was highest for early preterm birth (48.0 per 1000 deliveries), followed by late preterm birth (3.5), and term birth (0.5). Perinatal death occurred after 223 (9.1%) of 2456 iatrogenic preterm deliveries, compared with after 110 (2.6%) of 4266 spontaneous preterm births (P b 0.001). However, when intrauterine deaths and lethal fetal anomalies were excluded, the perinatal mortality did not differ between these two subtypes (31 [1.4%] of 2264 vs 65 [1.5%] of 4221; P = 0.67). Among the term deliveries (≥37 weeks), iatrogenic delivery was associated with a significantly higher corrected perinatal mortality than was spontaneous delivery (1.0 vs 0.4 per 1000 deliveries; P b 0.001) (Table 2). Among the 2456 iatrogenic preterm births, PPROM was the most common primary indication, followed by hypertensive disorders or
Fig. 1. Incidence of PTB and its clinical subtypes between 1995 and 2011. Abbreviations: PTB, preterm birth; PPROM, preterm premature rupture of membranes.
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Fig. 2. Percentage changes in the frequency of the clinical subtypes of PTB over the study period relative to levels in 1995. Abbreviations: PTB, preterm birth; PPROM, preterm premature rupture of membranes.
related complications, fetal compromise or distress, prepartum hemorrhage, intrauterine death, maternal disease, and fetal anomalies including lethal and non-lethal conditions (Table 3). Induction of labor for intrauterine deaths and lethal fetal anomalies was commonly performed as per departmental protocol; as a result, this group accounted for a large proportion of mortalities. Otherwise, clinical chorioamnionitis had the highest mortality risk of 83 perinatal deaths per 1000 deliveries, which was 10 times higher than that of PPROM without infection.
Fig. 1 shows the temporal trends in the overall frequency of preterm birth rate and its subtypes. The frequency of overall preterm birth was fairly consistent between 1995 and 2011, ranging between 5.5% and 7.2% (β = –0.39; P = 0.12) (Fig. 1). Early preterm birth fluctuated between 1.4% and 2.1% (β = –0.38; P = 0.13), and late preterm birth between 4.1% and 5.2% (β = –0.30; P = 0.25). Fig. 2 shows the changes in preterm birth relative to 1995 for the three clinical subtypes. Frequency of spontaneous preterm birth decreased significantly from 4.5% to 3.8%, equivalent to a 25% decrease
Fig. 3. Perinatal mortality rate for PTB and its clinical subtypes between 1995 and 2011. Abbreviations: PTB, preterm birth; PPROM, preterm premature rupture of membranes.
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(β = –0.83; P b 0.001). Preterm birth following PPROM increased significantly from 0.7% to 1.7%, equivalent to a 135% increase (β = 0.82; P b 0.001). The trend in iatrogenic preterm birth remained static, ranging from 1.1% to 1.9% (β = –0.22; P = 0.39). Fig. 3 shows the temporal trend in perinatal mortality for overall preterm birth and its subtypes from 1995 to 2011. The overall perinatal mortality rate decreased from 56.7 to 37.0 per 1000 deliveries before 37 weeks (β = –0.16; P = 0.54). When the subtypes were considered separately, there were no significant trends in mortality over the study period (Fig. 3). When the analysis was performed for 2002–2011 only, significant decreases were recorded for overall perinatal mortality (β = –0.70; P = 0.024) and perinatal mortality associated with iatrogenic preterm birth (β = –0.76; P =0.011). When intrauterine deaths and lethal fetal abnormalities were excluded from the analysis, neither trend remained significant.
4. Discussion Studies on the epidemiology of preterm birth among Chinese populations have been scarce [11]. The present study has shown that overall preterm rates for singletons in a Chinese population in Hong Kong have remained consistently low, with an overall frequency of 6.5% for 1995–2011. This value is lower than the most recent frequencies reported from the USA (9.9%) [2], Austria (8.7%) [12], Portugal (7.4%) [12], Germany (7.0%) [12], and Australia (6.7%) [13], but higher than those reported from 16 European countries (4.3%–6.3%) [11] and England, Wales, and Scotland (5.6%) [14,15]. In the present cohort, the frequency of overall preterm birth remained fairly static over the study period, as did the incidence of early and late preterm birth. With advances in obstetric and neonatal care, some studies have reported an increase in late preterm birth in particular, precipitated by a fairly low risk of perinatal mortality for neonates delivered after 34 weeks and thus a lowered threshold for obstetric intervention that might not be medically indicated [16]; however, this was not observed in the present cohort. Furthermore, no significant change was observed in the trend of iatrogenic preterm birth. The most notable observation in the present study was the decreasing trend in spontaneous preterm birth. This decrease may reflect improved nutrition and environmental factors among Hong Kong women of childbearing age [17], in addition to better healthcare standards, including earlier risk detection, provision of effective preventive measures (e.g. use of cerclage and progesterone), and use of tocolysis in high-risk cases [18,19]. By contrast, preterm birth following PPROM increased significantly over the same period, which offset the decrease in spontaneous preterm birth. It seems likely that, although early identification has allowed intervention in pregnancies that might have ended in spontaneous preterm birth, PPROM has remained unpreventable. This observation is compatible with the notion of an “inherent” preterm rate—i.e., for a certain population, the combined spontaneous preterm birth, iatrogenic preterm birth, and preterm birth following rupture of membranes will add up to a constant value that will not be mitigated by changing the threshold of medical intervention because high-risk pregnancies will eventually end in preterm birth one way or another. A similar observation was made by Ananth et al. [5], who concluded that “the different subtypes of preterm birth share common determinants, and hence an increase in a particular subtype may result in a decrease in another.” Because preterm birth is a complex subject and its etiologies are multifactorial, the limited measures that are currently available may be inadequate to reduce the inherent preterm rate. Genetic and socioeconomic factors may affect this rate in various communities. Epidemiologic studies have shown that some problems are less prevalent among Chinese mothers—e.g., only 2.3% of women in the database used in the present study were shown to be obese according to the WHO’s classification [20] and smoking in pregnancy was found infrequently [21]—and
these variations in risk factors may have different effects on the occurrence of preterm birth in the study population. The present study also confirmed that preterm birth was a significant contributor to total perinatal mortality (early preterm birth contributed 16.0% and late preterm birth 3.4%), especially early preterm birth. Of note, the greatest contributor to overall perinatal mortality was intrauterine deaths and lethal fetal anomalies (70.6%), whereas perinatal mortality among term pregnancies accounted for 9.9%. Notably, a decrease in the number of intrauterine deaths and lethal fetal abnormalities led to an overall lower perinatal mortality rate in the past decade. To reduce the perinatal mortality rate further, future efforts should be directed toward lowering both the preterm rate and the number of intrauterine deaths and fetal anomalies. A strength of the present study was that it was confined to singletons. Many studies that have shown increases in preterm birth in other countries over the past two decades, including the WHO global survey [3], have included singletons and multiple pregnancies together. However, singletons and multiple pregnancies are distinct entities, particularly in the context of preterm birth and perinatal outcomes [22]. Preterm birth is much more common for multiple pregnancies (affecting 40% to 70%) than for singleton pregnancies [23]. With the increasing availability and popularity of assisted reproductive technologies, the numbers of multiple pregnancies are growing; thus, there is a need to examine them separately to avoid drawing the wrong conclusions. Another strength of the present study is that different subtypes of preterm birth were examined in detail and the indications for iatrogenic preterm birth were described, thereby covering the entire preterm spectrum. Nevertheless, this method of classifying preterm birth is not perfect—e.g. some women with spontaneous onset of labor who subsequently delivered due to a separate medical indication might be counted as iatrogenic birth. The present study has several limitations. First, the study covered a long period, and therefore women with more than one pregnancy are likely to be included multiple times. The database available for analysis did not contain patient identifiers to allow an assessment of a specific patient's individual pregnancies, so no corrections for this factor was applied. Second, comparison of the present study with others may be difficult, because the definition for viability varies among countries: some use a gestation of 22 weeks, 24 weeks, or 28 weeks; and others use a birth weight of more than 500 g [24]. Third, owing to resource limitations, an early dating scan was not offered to every woman, and some might have had a late first prenatal visit which might affect the accuracy of the gestational age estimations. Fourth, because the study was referral center-based instead of population-based, the cohort might have been biased toward a higher-risk subpopulation such that the perinatal mortality rate might have been higher than that of the general population in Hong Kong. Last, perinatal mortality is only one indicator of the quality of obstetric health care, and the current database did not allow long-term morbidity among the survivors to be traced. In Hong Kong, there have previously been no systematic territorywide annual figures for the rates of preterm birth other than those reported by the Hong Kong College of Obstetricians and Gynaecologists after its territory-wide audits in 1994, 1999, 2004, and 2009. The observed rates of preterm delivery in the present study were in agreement with those reported in these audits [25]. Furthermore, because the study unit is a tertiary center to which women are referred for specialist care, the data generated in the study unit are likely to be representative of the worst situation. Thus, the effect of changing obstetric practice on the incidence of preterm birth and related perinatal mortality might be increased in the study unit. In conclusion, the present study has shown that the overall preterm birth rate for singletons in this Hong Kong population remained consistent from 1995 to 2011. Although spontaneous preterm birth decreased significantly, it was offset by a simultaneous increase in preterm birth following PPROM. The overall perinatal mortality rate has gradually fallen, and its main contributors are preterm birth, intrauterine death, and
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fetal anomalies. The present observations warrant further studies to optimize pregnancy outcomes among women at increased risk of preterm birth. Conflict of interest The authors have no conflicts of interest. References [1] Blencowe H, Cousens S, Oestergaard MZ, Chou D, Moller AB, Narwal R, et al. National, regional, and worldwide estimates of preterm birth rates in the year 2010 with time trends since 1990 for selected countries: a systematic analysis and implications. Lancet 2012;379(9832):2162–72. [2] Martin JA, Hamilton BE, Osterman MJK, Curtin SC, Mathews TJ. Births: final data for 2012. http://www.cdc.gov/nchs/data/nvsr/nvsr62/nvsr62_09.pdf. Published December 30, 2013. Accessed July 31, 2014. [3] Liu L, Johnson HL, Cousens S, Perin J, Scott S, Lawn JE, et al. Global, regional, and national causes of child mortality: an updated systematic analysis for 2010 with time trends since 2000. Lancet 2012;379(9832):2151–61. [4] Newnham JP, Sahota DS, Zhang CY, Xu B, Zheng M, Doherty DA, et al. Preterm birth rates in Chinese women in China, Hong Kong and Australia—the price of Westernisation. Aust N Z J Obstet Gynaecol 2011;51(5):426–31. [5] Ananth CV, Joseph KS, Oyelese Y, Demissie K, Vintzileos AM. Trends in preterm birth and perinatal mortality among singletons: United States, 1989 through 2000. Obstet Gynecol 2005;105(5 Pt 1):1084–91. [6] Lisonkova S, Hutcheon JA, Joseph KS. Temporal trends in neonatal outcomes following iatrogenic preterm delivery. BMC Pregnancy Childbirth 2011;11:39. [7] Hong Kong Census and Statistics Department. Ethnic minorities by ethnicity and age group, 2001, 2006 and 2011 (F401). http://www.census2011.gov.hk/en/main-table/ F401.html. Published February 22, 2013. Accessed July 31, 2014. [8] Royal College of Obstetricians and Gynaecologists. Preterm Prelabour Rupture of Membranes: Green-top Guideline No. 44. http://www.rcog.org.uk/files/rcog-corp/ GTG44PPROM28022011.pdf. Published November 2006. Updated October 2010. Accessed July 31, 2014. [9] Barfield WD. Committee on Fetus and Newborn. Standard terminology for fetal, infant, and perinatal deaths. Pediatrics 2011;128(1):177–81. [10] Committee on Obstetric Practice. ACOG Committee opinion: perinatal and infant mortality statistics. Int J Gynecol Obstet 1996;53(1):86–8.
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International Journal of Gynecology and Obstetrics journal homepage: www.elsevier.com/locate/ijgo
CLINICAL ARTICLE
Trends in preterm birth in singleton deliveries in a Hong Kong population Annie S.Y. Hui a,⁎, Terence T. Lao a, Tak Yeung Leung a, Jelle M. Schaaf b, Daljit S. Sahota a a b
Department of Obstetrics and Gynaecology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong Special Administrative Region Department of Medical Informatics, Academic Medical Centre, Amsterdam, Netherlands
a r t i c l e
i n f o
Article history: Received 22 January 2014 Received in revised form 23 June 2014 Accepted 1 August 2014 Keywords: Perinatal mortality Preterm birth Preterm premature rupture of membranes
a b s t r a c t Objective: To examine trends in preterm birth and its relationship with perinatal mortality in Hong Kong. Methods: In a retrospective cohort study, data were reviewed from singletons delivered between 1995 and 2011 at a university teaching hospital. Trends in preterm birth (between 24 and 36 weeks of pregnancy), perinatal mortality, and subtypes of preterm birth (spontaneous, iatrogenic, and following preterm premature rupture of membranes [PPROM]) were examined via linear regression. Results: There were 103 364 singleton deliveries, of which 6722 (6.5%) occurred preterm, including 1835 (1.8%) early preterm births (24–33 weeks) and 4887 (4.7%) late preterm births (34–36 weeks). Frequency of preterm birth remained fairly consistent over the study period, but that of spontaneous preterm birth decreased by 25% (β = –0.83; P b 0.001), from 4.5% to 3.8%. Frequency of preterm birth following PPROM increased by 135% (β = 0.82; P b 0.001), from 0.7% to 1.7%. The perinatal mortality rate decreased from 56.7 to 37.0 deaths per 1000 deliveries before 37 weeks (β = –0.16; P = 0.54). Early preterm birth contributed to 16.0% of all deaths. Conclusion: Although the overall rate of preterm birth in Hong Kong has remained constant, the frequencies of its subtypes have changed. Overall perinatal mortality is gradually decreasing, but early preterm birth remains a major contributor. © 2014 International Federation of Gynecology and Obstetrics. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Despite the increased accessibility and improved standards of prenatal care in most countries, the incidence of preterm birth has continued to rise over the past two decades in some nations [1]. Late preterm birth (at 34–36 weeks of gestation) is still the most common type, accounting for up to 72% of all preterm births in the USA in 2012 [2]. Although survival of preterm newborns has greatly improved because of advances in neonatal intensive care, the social and economic burdens of preterm birth remain substantial. Preterm-related complications are estimated to account for more than 35% of the 3.1 million neonatal deaths that occur worldwide every year, and preterm birth is still the second most common cause of death among children younger than 5 years [3]. In Asia, the estimated incidence of preterm birth varies from 7.2% in East Asia to 13.6% in Southeast Asia [1]. The reason for the differences among Asian regions is unclear, but a comparison between Chinese women living in China, Hong Kong, and Australia showed that the frequency of preterm birth was higher for those residing in high-income regions [4]. This variation might be partly related to increased medical interventions. In the USA, for example, an increase in the frequency of preterm birth was caused mainly by a rise in the number of medically
⁎ Corresponding author at: 1E, Department of Obstetrics and Gynecology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong Special Administrative Region. Tel.: +852 26322211; fax: +852 26360008. E-mail address: [email protected] (A.S.Y. Hui).
indicated deliveries, whereas spontaneous preterm birth decreased [5,6]; once healthcare providers became aware of the problem, efforts were successfully made to reduce the preterm rates from 11.1% in 2006 to 9.9% in 2012 [2]. Nevertheless, the overall frequency of preterm birth provides no information on the role of medical interventions or on the relative contributions of spontaneous versus iatrogenic preterm birth to the perinatal mortality rate among preterm neonates. Hong Kong lies between low-resource and high-resource countries in terms of economic development. In 2011, 93.6% of the 7.07 million individuals in Hong Kong were of Chinese ethnic origin [7]. Therefore, studies in Hong Kong of the trends in preterm birth and their impact on perinatal mortality would not be confounded by variation in ethnic origin due to population migration and immigration, and would help to elucidate the influence of changing obstetric practice and the associated perinatal mortality so that appropriate strategies for preventing preterm birth and reducing perinatal mortality can be devised. It is also informative to examine the trends in early and late preterm birth, and in the three clinical subtypes (spontaneous preterm birth, iatrogenic preterm birth, and preterm births following preterm premature rupture of membranes [PPROM]). It is helpful to consider preterm birth following PPROM as a stand-alone subtype because—according to the latest Royal College of Obstetricians and Gynaecologists guideline for PPROM [8]—induction after PPROM is recommended at or after 34 weeks if there is no spontaneous onset of labor. Thus, preterm birth following PPROM includes both spontaneous and iatrogenic births. The primary aim of the present study was to examine trends in preterm birth and associations with perinatal mortality among singleton
http://dx.doi.org/10.1016/j.ijgo.2014.06.019 0020-7292/© 2014 International Federation of Gynecology and Obstetrics. Published by Elsevier Ireland Ltd. All rights reserved.
A.S.Y. Hui et al. / International Journal of Gynecology and Obstetrics 127 (2014) 248–253 Table 1 Frequency of preterm birth among 103 364 singleton deliveries. Type
No. (%)
Preterm birtha Early preterm birthb Late preterm birthc Spontaneous preterm birth Spontaneous preterm birth after PPROM Iatrogenic preterm birth Iatrogenic preterm birth after PPROM
6722 (6.5) 1835 (1.8) 4887 (4.7) 4266 (4.1) 338 (0.3) 2456 (2.4) 754 (0.7)
Abbreviation: PPROM, premature rupture of membrane. a Between 24 and 36 weeks, 6 days. b Between 24 and 33 weeks, 6 days. c Between 34 and 36 weeks, 6 days.
deliveries in Hong Kong. A secondary aim was to carry out a subgroup analysis for iatrogenic preterm birth to examine common indications and their associated perinatal mortality risk.
2. Materials and methods In a retrospective cohort study, data were reviewed from all singletons delivered between January 1, 1995, and October 31, 2011, at the Prince of Wales Hospital in Hong Kong—a university obstetrics unit taking care of both high- and low-risk women, with more than 6000 deliveries per year. Ethics approval was obtained from the Joint Chinese University of Hong Kong–New Territories East Cluster Clinical Research Ethics Committee (reference number 2012.256). Informed consent was not required because the data did not contain any identifying information. The study data were obtained from the Obstetric Specialty Clinical Information System database, which was set up by the Hong Kong Hospital Authority to record all maternal and perinatal outcomes in public hospitals. For this database, prenatal information and perinatal outcomes are prospectively recorded at each visit and at the time of delivery by the attending midwife. The accuracy and validity of the data are checked by a second midwife. Moreover, perinatal outcomes are reviewed in monthly and annual audit meetings to ensure that all adverse events were identified. Gestational age was estimated on the basis of the date of last menstrual period and/or crown–rump length measurement during early dating ultrasound. A service for anomaly scans in the second trimester was introduced in 1997, and combined Down syndrome screening in the first trimester in 2003. Therefore, most women who delivered in the study period had had at least one prenatal scan. These scans were
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used to revise the estimated delivery date and gestational age if they were inconsistent with those calculated from the last menstrual period. Preterm birth was defined as delivery between the start of week 24 and the end of week 36. Early preterm birth occurred between the start of week 24 and the end of week 33, and late preterm birth occurred between the start of week 34 and the end of week 36. Pregnancies that ended before week 24, including spontaneous abortions and medically or surgically terminated pregnancies, were excluded. Preterm birth was classified into three subtypes: spontaneous preterm birth (birth following spontaneous onset of labor), iatrogenic preterm birth (medically indicated deliveries, with either induction of labor or cesarean), and preterm birth following PPROM (including both spontaneous and iatrogenic births). Perinatal death was defined according to the US National Centre of Health Statistics, American Academy of Pediatrics Committee on Fetus and Newborn, and the American College of Obstetricians and Gynecologists Committee on Obstetric Practice [9,10]. The minimum gestational age was adjusted according to the legal definition of viability in Hong Kong (24 weeks’ gestation). Therefore, in the present study, the overall perinatal mortality rate was defined as the sum of fetal deaths (≥ 24 weeks of gestation) plus neonatal deaths within the first 28 days of delivery, expressed per 1000 deliveries. Specific mortality risks were calculated per 1000 deliveries of the specified gestation or clinical condition. The absolute and relative trends in the incidence of preterm birth overall and in the three subtypes, and in overall perinatal mortality were examined. Intrauterine deaths and lethal fetal anomalies were common confounders, because induction of labor was typically offered before term in these cases in the study cohort, and so they would be counted as iatrogenic preterm births in most cases. Therefore, in addition to calculating overall perinatal mortality, the analysis was repeated without these cases, thereby generating a corrected perinatal mortality rate to reflect more accurately the contribution of preterm birth to perinatal mortality. Perinatal mortality for iatrogenic preterm birth was also examined. Common indications were categorized and the specific mortality risk was calculated. Statistical analysis was performed via PASW Statistics for Windows version 18.0 (IBM, Armonk, NY, USA). Descriptive analysis was used to calculate the prevalence of preterm birth. Trend analysis was undertaken by performing a linear-by-linear trend test, and then by fitting a linear regression model for each series and testing the regression coefficient (β) to see whether there was significant deviation from zero via a t test. For comparisons between groups, χ2 tests were used as appropriate. All statistical tests were two-tailed. P b 0.05 was considered significant.
Table 2 Preterm births and related mortality.a Gestational age at delivery
24–33 weeks Total Iatrogenic Spontaneous 34–36 weeks Total Iatrogenic Spontaneous ≥37 weeks Total Iatrogenic Spontaneous a b c d
Deliveries (n = 103 364)
Perinatal deaths (n = 493)b
Overall perinatal mortality rate (per 1000 deliveries)
Intrauterine deaths and lethal anomalies (n = 348)c
1835 (1.8) 698 (0.7) 1137 (1.1)
268 (54.4) 178 (36.1) 90 (18.3)
146.0 255.0 79.2
189 (54.3) 156 (44.8) 33 (9.5)
4887 (4.7) 1758 (1.7) 3129 (3.0)
65 (13.2) 45 (9.1) 20 (4.1)
13.3 25.6 6.4
96 642 (93.5) 23 999 (23.2) 72 643 (70.3)
160 (32.5) 89 (18.1) 71 (14.4)
1.7 3.7 1.0
Corrected number of deliveries (n = 103 016)
Corrected number of perinatal deaths (n = 145)
Corrected perinatal mortality rate (per 1000 deliveries)
Corrected contribution to overall mortality (%)d
1646 (1.6) 542 (0.5) 1104 (1.1)
79 (54.5) 22 (15.2) 57 (39.3)
48.0 40.6 51.6
16.0 4.5 11.6
48 (13.8) 36 (10.3) 12 (3.4)
4839 (4.7) 1722 (1.7) 3117 (3.0)
17 (11.7) 9 (6.2) 8 (5.5)
3.5 5.2 2.6
3.4 1.8 1.6
111 (31.9) 66 (19.0) 45 (12.9)
96 531 (93.7) 23 933 (23.2) 72 598 (70.5)
49 (33.8) 23 (15.9) 26 (17.9)
0.5 1.0 0.4
9.9 4.7 5.3
Values are given as number (percentage) unless otherwise indicated. Fetal deaths (≥24 gestational weeks) plus neonatal deaths within the first 28 days of birth. Intrauterine deaths were intrauterine fetal deaths at a gestational age of less than 24 weeks. Percentages calculated with total number of perinatal deaths to reflect overall contribution to mortality.
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3. Results
Table 3 Causes of iatrogenic preterm birth and associated perinatal mortality risks.a Cause
Gestational age, wk
Iatrogenic Perinatal preterm deaths births (n = 223) (n = 2456)
PPROM Clinical chorioamnionitis Hypertensive disorders in pregnancy Severe pre-eclampsia Eclampsia Placental abruption Fetal compromise Fetal compromise (primary) Intrauterine growth restriction Fetal distress Suboptimal CTG Oligohydramnios Cord incidents Prepartum hemorrhage Placenta previa Placental abruption Not specified Maternal diseases Maternal compromise Fetal complications Poor obstetrics history Intrauterine death Fetal anomaly
34.98 ± 1.49 754 (30.7) 32.35 ± 3.20 72 (2.9) 33.19 ± 2.78 548 (22.3)
6 (2.7) 6 (2.7) 7 (3.1)
8.0 83.3 12.9
33.43 32.71 31.50 32.09 34.35 33.97
± ± ± ± ± ±
2.74 446 (18.2) 3.45 7 (0.3) 1.87 6 (0.2) 2.70 89 (3.6) 2.25 407 (16.6) 2.31 154 (6.3)
5 (2.2) 0 0 2 (0.9) 8 (3.6) 2 (0.9)
11.2
33.64 35.15 35.67 33.89 34.16 34.23 32.91 34.53 34.62 34.57 33.57 36.00 29.70 32.56
± ± ± ± ± ± ± ± ± ± ± ± ± ±
2.62 1.27 0.67 3.16 2.27 2.01 2.92 2.62 2.12 2.18 1.81 0.00 3.76 2.85
4 (1.8) 1 (0.4) 1 (0.4) 0 3 (1.3) 2 (0.9) 1 (0.4) 0 1 (0.4) 1 (0.4) 0 0 156 (70.0) 36 (16.1)
107 (4.4) 67 (2.7) 64 (2.6) 15 (0.6) 353 (14.4) 249 (10.1) 34 (1.4) 70 (2.9) 89 (3.6) 74 (3.0) 7 (0.3) 8 (0.3) 156 (6.4) 77 (3.1)
Perinatal mortality (per 1000 deliveries)
22.5 19.7 13.0 37.4 14.9 15.6 8.5 8.0 29.4 11.2 13.5
1000 467.5
Abbreviation: PPROM, preterm premature rupture of membranes. a Values are given as mean ± SD or number (percentage), unless otherwise indicated.
Over the 17-year study period, there were 103 364 singleton deliveries, among which 99 457 (96.2%) mothers were of Chinese ethnic origin. Overall, 6722 (6.5%) births occurred preterm, including 1835 (1.8%) early preterm births and 4887 (4.7%) late preterm births (Table 1). Spontaneous preterm birth was recorded for 4266 (4.1%) deliveries, iatrogenic preterm birth for 2456 (2.4%), and preterm birth after PPROM for 1092 (1.1%). A total of 493 perinatal deaths occurred, giving an overall perinatal mortality rate of 4.77 deaths per 1000 deliveries. Table 2 shows the contribution of preterm birth to perinatal mortality. After excluding 348 intrauterine deaths and lethal fetal anomalies, the corrected perinatal mortality rate was 1.4 per 1000 deliveries. Of the 145 perinatal deaths that were not intrauterine or related to lethal anomalies, 79 (54.5%) occurred after early preterm birth and 17 (11.7%) after late preterm birth. The mortality rate was highest for early preterm birth (48.0 per 1000 deliveries), followed by late preterm birth (3.5), and term birth (0.5). Perinatal death occurred after 223 (9.1%) of 2456 iatrogenic preterm deliveries, compared with after 110 (2.6%) of 4266 spontaneous preterm births (P b 0.001). However, when intrauterine deaths and lethal fetal anomalies were excluded, the perinatal mortality did not differ between these two subtypes (31 [1.4%] of 2264 vs 65 [1.5%] of 4221; P = 0.67). Among the term deliveries (≥37 weeks), iatrogenic delivery was associated with a significantly higher corrected perinatal mortality than was spontaneous delivery (1.0 vs 0.4 per 1000 deliveries; P b 0.001) (Table 2). Among the 2456 iatrogenic preterm births, PPROM was the most common primary indication, followed by hypertensive disorders or
Fig. 1. Incidence of PTB and its clinical subtypes between 1995 and 2011. Abbreviations: PTB, preterm birth; PPROM, preterm premature rupture of membranes.
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Fig. 2. Percentage changes in the frequency of the clinical subtypes of PTB over the study period relative to levels in 1995. Abbreviations: PTB, preterm birth; PPROM, preterm premature rupture of membranes.
related complications, fetal compromise or distress, prepartum hemorrhage, intrauterine death, maternal disease, and fetal anomalies including lethal and non-lethal conditions (Table 3). Induction of labor for intrauterine deaths and lethal fetal anomalies was commonly performed as per departmental protocol; as a result, this group accounted for a large proportion of mortalities. Otherwise, clinical chorioamnionitis had the highest mortality risk of 83 perinatal deaths per 1000 deliveries, which was 10 times higher than that of PPROM without infection.
Fig. 1 shows the temporal trends in the overall frequency of preterm birth rate and its subtypes. The frequency of overall preterm birth was fairly consistent between 1995 and 2011, ranging between 5.5% and 7.2% (β = –0.39; P = 0.12) (Fig. 1). Early preterm birth fluctuated between 1.4% and 2.1% (β = –0.38; P = 0.13), and late preterm birth between 4.1% and 5.2% (β = –0.30; P = 0.25). Fig. 2 shows the changes in preterm birth relative to 1995 for the three clinical subtypes. Frequency of spontaneous preterm birth decreased significantly from 4.5% to 3.8%, equivalent to a 25% decrease
Fig. 3. Perinatal mortality rate for PTB and its clinical subtypes between 1995 and 2011. Abbreviations: PTB, preterm birth; PPROM, preterm premature rupture of membranes.
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(β = –0.83; P b 0.001). Preterm birth following PPROM increased significantly from 0.7% to 1.7%, equivalent to a 135% increase (β = 0.82; P b 0.001). The trend in iatrogenic preterm birth remained static, ranging from 1.1% to 1.9% (β = –0.22; P = 0.39). Fig. 3 shows the temporal trend in perinatal mortality for overall preterm birth and its subtypes from 1995 to 2011. The overall perinatal mortality rate decreased from 56.7 to 37.0 per 1000 deliveries before 37 weeks (β = –0.16; P = 0.54). When the subtypes were considered separately, there were no significant trends in mortality over the study period (Fig. 3). When the analysis was performed for 2002–2011 only, significant decreases were recorded for overall perinatal mortality (β = –0.70; P = 0.024) and perinatal mortality associated with iatrogenic preterm birth (β = –0.76; P =0.011). When intrauterine deaths and lethal fetal abnormalities were excluded from the analysis, neither trend remained significant.
4. Discussion Studies on the epidemiology of preterm birth among Chinese populations have been scarce [11]. The present study has shown that overall preterm rates for singletons in a Chinese population in Hong Kong have remained consistently low, with an overall frequency of 6.5% for 1995–2011. This value is lower than the most recent frequencies reported from the USA (9.9%) [2], Austria (8.7%) [12], Portugal (7.4%) [12], Germany (7.0%) [12], and Australia (6.7%) [13], but higher than those reported from 16 European countries (4.3%–6.3%) [11] and England, Wales, and Scotland (5.6%) [14,15]. In the present cohort, the frequency of overall preterm birth remained fairly static over the study period, as did the incidence of early and late preterm birth. With advances in obstetric and neonatal care, some studies have reported an increase in late preterm birth in particular, precipitated by a fairly low risk of perinatal mortality for neonates delivered after 34 weeks and thus a lowered threshold for obstetric intervention that might not be medically indicated [16]; however, this was not observed in the present cohort. Furthermore, no significant change was observed in the trend of iatrogenic preterm birth. The most notable observation in the present study was the decreasing trend in spontaneous preterm birth. This decrease may reflect improved nutrition and environmental factors among Hong Kong women of childbearing age [17], in addition to better healthcare standards, including earlier risk detection, provision of effective preventive measures (e.g. use of cerclage and progesterone), and use of tocolysis in high-risk cases [18,19]. By contrast, preterm birth following PPROM increased significantly over the same period, which offset the decrease in spontaneous preterm birth. It seems likely that, although early identification has allowed intervention in pregnancies that might have ended in spontaneous preterm birth, PPROM has remained unpreventable. This observation is compatible with the notion of an “inherent” preterm rate—i.e., for a certain population, the combined spontaneous preterm birth, iatrogenic preterm birth, and preterm birth following rupture of membranes will add up to a constant value that will not be mitigated by changing the threshold of medical intervention because high-risk pregnancies will eventually end in preterm birth one way or another. A similar observation was made by Ananth et al. [5], who concluded that “the different subtypes of preterm birth share common determinants, and hence an increase in a particular subtype may result in a decrease in another.” Because preterm birth is a complex subject and its etiologies are multifactorial, the limited measures that are currently available may be inadequate to reduce the inherent preterm rate. Genetic and socioeconomic factors may affect this rate in various communities. Epidemiologic studies have shown that some problems are less prevalent among Chinese mothers—e.g., only 2.3% of women in the database used in the present study were shown to be obese according to the WHO’s classification [20] and smoking in pregnancy was found infrequently [21]—and
these variations in risk factors may have different effects on the occurrence of preterm birth in the study population. The present study also confirmed that preterm birth was a significant contributor to total perinatal mortality (early preterm birth contributed 16.0% and late preterm birth 3.4%), especially early preterm birth. Of note, the greatest contributor to overall perinatal mortality was intrauterine deaths and lethal fetal anomalies (70.6%), whereas perinatal mortality among term pregnancies accounted for 9.9%. Notably, a decrease in the number of intrauterine deaths and lethal fetal abnormalities led to an overall lower perinatal mortality rate in the past decade. To reduce the perinatal mortality rate further, future efforts should be directed toward lowering both the preterm rate and the number of intrauterine deaths and fetal anomalies. A strength of the present study was that it was confined to singletons. Many studies that have shown increases in preterm birth in other countries over the past two decades, including the WHO global survey [3], have included singletons and multiple pregnancies together. However, singletons and multiple pregnancies are distinct entities, particularly in the context of preterm birth and perinatal outcomes [22]. Preterm birth is much more common for multiple pregnancies (affecting 40% to 70%) than for singleton pregnancies [23]. With the increasing availability and popularity of assisted reproductive technologies, the numbers of multiple pregnancies are growing; thus, there is a need to examine them separately to avoid drawing the wrong conclusions. Another strength of the present study is that different subtypes of preterm birth were examined in detail and the indications for iatrogenic preterm birth were described, thereby covering the entire preterm spectrum. Nevertheless, this method of classifying preterm birth is not perfect—e.g. some women with spontaneous onset of labor who subsequently delivered due to a separate medical indication might be counted as iatrogenic birth. The present study has several limitations. First, the study covered a long period, and therefore women with more than one pregnancy are likely to be included multiple times. The database available for analysis did not contain patient identifiers to allow an assessment of a specific patient's individual pregnancies, so no corrections for this factor was applied. Second, comparison of the present study with others may be difficult, because the definition for viability varies among countries: some use a gestation of 22 weeks, 24 weeks, or 28 weeks; and others use a birth weight of more than 500 g [24]. Third, owing to resource limitations, an early dating scan was not offered to every woman, and some might have had a late first prenatal visit which might affect the accuracy of the gestational age estimations. Fourth, because the study was referral center-based instead of population-based, the cohort might have been biased toward a higher-risk subpopulation such that the perinatal mortality rate might have been higher than that of the general population in Hong Kong. Last, perinatal mortality is only one indicator of the quality of obstetric health care, and the current database did not allow long-term morbidity among the survivors to be traced. In Hong Kong, there have previously been no systematic territorywide annual figures for the rates of preterm birth other than those reported by the Hong Kong College of Obstetricians and Gynaecologists after its territory-wide audits in 1994, 1999, 2004, and 2009. The observed rates of preterm delivery in the present study were in agreement with those reported in these audits [25]. Furthermore, because the study unit is a tertiary center to which women are referred for specialist care, the data generated in the study unit are likely to be representative of the worst situation. Thus, the effect of changing obstetric practice on the incidence of preterm birth and related perinatal mortality might be increased in the study unit. In conclusion, the present study has shown that the overall preterm birth rate for singletons in this Hong Kong population remained consistent from 1995 to 2011. Although spontaneous preterm birth decreased significantly, it was offset by a simultaneous increase in preterm birth following PPROM. The overall perinatal mortality rate has gradually fallen, and its main contributors are preterm birth, intrauterine death, and
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fetal anomalies. The present observations warrant further studies to optimize pregnancy outcomes among women at increased risk of preterm birth. Conflict of interest The authors have no conflicts of interest. References [1] Blencowe H, Cousens S, Oestergaard MZ, Chou D, Moller AB, Narwal R, et al. National, regional, and worldwide estimates of preterm birth rates in the year 2010 with time trends since 1990 for selected countries: a systematic analysis and implications. Lancet 2012;379(9832):2162–72. [2] Martin JA, Hamilton BE, Osterman MJK, Curtin SC, Mathews TJ. Births: final data for 2012. http://www.cdc.gov/nchs/data/nvsr/nvsr62/nvsr62_09.pdf. Published December 30, 2013. Accessed July 31, 2014. [3] Liu L, Johnson HL, Cousens S, Perin J, Scott S, Lawn JE, et al. Global, regional, and national causes of child mortality: an updated systematic analysis for 2010 with time trends since 2000. Lancet 2012;379(9832):2151–61. [4] Newnham JP, Sahota DS, Zhang CY, Xu B, Zheng M, Doherty DA, et al. Preterm birth rates in Chinese women in China, Hong Kong and Australia—the price of Westernisation. Aust N Z J Obstet Gynaecol 2011;51(5):426–31. [5] Ananth CV, Joseph KS, Oyelese Y, Demissie K, Vintzileos AM. Trends in preterm birth and perinatal mortality among singletons: United States, 1989 through 2000. Obstet Gynecol 2005;105(5 Pt 1):1084–91. [6] Lisonkova S, Hutcheon JA, Joseph KS. Temporal trends in neonatal outcomes following iatrogenic preterm delivery. BMC Pregnancy Childbirth 2011;11:39. [7] Hong Kong Census and Statistics Department. Ethnic minorities by ethnicity and age group, 2001, 2006 and 2011 (F401). http://www.census2011.gov.hk/en/main-table/ F401.html. Published February 22, 2013. Accessed July 31, 2014. [8] Royal College of Obstetricians and Gynaecologists. Preterm Prelabour Rupture of Membranes: Green-top Guideline No. 44. http://www.rcog.org.uk/files/rcog-corp/ GTG44PPROM28022011.pdf. Published November 2006. Updated October 2010. Accessed July 31, 2014. [9] Barfield WD. Committee on Fetus and Newborn. Standard terminology for fetal, infant, and perinatal deaths. Pediatrics 2011;128(1):177–81. [10] Committee on Obstetric Practice. ACOG Committee opinion: perinatal and infant mortality statistics. Int J Gynecol Obstet 1996;53(1):86–8.
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