N.Z.J. ObJtet. Gytitrec. (1979) 19: 77
Routine Fetal Maturity Estimation
G. B. Duff' Departn letit of Obstetrics mid Gynuecolog y , Cliristchurch Clitiiccil School of Medicine, CliriJtcliurcli, New Zealund
Sur?it?iury:The accuracy of ultrasonic estimation of fetal maturity and, therefore, the delivery date is greatest when the biparietal diameter is less than 5.0 cm and least when it is greater than 9.0 cm. A comparison of the accuracy of ultrasonic and last menstrual period based estimations of the delivery date revealed that there is no significant increase in accuracy for ultrasonic estimations when the biparietal diameter is less than 9.0cm, but when the diameter is greater than 9.0 cni the accuracy of delivery date estimation is siznificantly less than that based on the last menstrual period. Tn pregnancies complicated by uncertain dates, recent hormonal contraception, bleeding in early pregnancy, or an irregular or prolonged menstrual cycle, ultrasound is significantly more accurate in predicting the delivery date. Ultrasonic examination for fetal maturity is indicated only when there are clinical indications such as uncertain dates for some reason or when there is fundal height discrepancy.
Ultrasonic examination of every pregnancy on at least one occasion has been recommended principally to check on the period of gestation, but also to check on fetal growth and placental position and to exclude multiple pregnancy and some fetal a bnor ma 1 it i es. Previous studies by Campbell (1969), Varma (1973), and Robinson and Flemming (1975), have shown that there is a hi$ degree of accuracy in predicting the delivery date from fetal nieasurement. However, these studies have been performed on populations which show a small standard deviation about the mean. Recent studies of maximum fetal diameter and biparietal diameter measurements in a New Zealand population (Aickin et al., 1976; Dutf, 1979) have shown a significantly larger standard deviation about the mean at almost all gestations. The aim of the present study is to ascertain the accurary of fetal measurement in prcdicting the I . Senior Lecturer.
delivery date in a New Zealand population and to ascertain in which groups of patients ultrasonic examination is of benefit. PATIENTS A N D METHODS
A detailed menstrual history was obtained from 257 patients who booked before 20 weeks of gestation. This was facilitated by redesigning the antenatal case sheet to obtain the following information: The date of the last menstrual period. Whether this date was known with certainty. Thc timing, duration and amount of the last menstrual period. T h e normal cycle length and duration of menstruation. Whether hormonal contraception had been used within 3 months of the last menstrual period; if so, the type and date of cessation. Ultrasonic examinations were made using a Nuclear Enterprises Diasonograph (NE 4102) with Grey Scale ( N E 4104). The patients were all scanned soon after booking and within the first
AUST.AND N.Z. JOURNAL
20 weeks of gestation. The ultrasonic examinations were repeated again on a number of occasions after 28 weeks of gestation. The maximum fetal diameters were measured by a technique described by Higginbottom et al. (1977), and the biparietal diameters by the technique described by Campbell (1968). RESULTS
Amongst the 257 patients whose menstrual data, delivery date and ultrasonic examinations were analysed, 935 ultrasonic examinations for fetal measurement were performed. However, 82 (31.9%) had either an induction of labour or an elective Caesarean section and although 17 (6.6%) of those inductions were for prolonged pregnancy these cases have been excluded. In the remaining 175 patients who laboured spontaneously, 590 ultrasonic examinations were performed.
Sure of Dates Group Of the 175 patients, 137 (78.3%) were sure of the date of the last menstrual period. Eighty-nine (65%) delivered within 10 days of the calculated delivery date and 103 (75.2%) delivered within 14 days. (a) Eurly Ultrasound Exurnination One hundred and ninety ultrasonic exxaminations were performed before 20 weeks of gestation in the 137 patients who were sure of the date of the last menstrual period and who laboured spontaneously. Calculations from these measurements of maximum fetal diameter and biparietal diameter based on the figures derived from a similar population (Duff, 1979) showed that the calculated delivery date was within 10 days of the actual delivery date in 68.9% and within 14 days in 82.6%. These differences between the last menstrual period and ultrasound based calculations were not statistically significant. Of the 190 early ultrasonic examinations 51 were measurements of the maximum fetal diameter and 139 were biparietal diameter measurements less than 5.0 cm. Calculations of the delivery date based on the maximum fetal diameter measurements revealed that 64.7% delivered within 10 days of the calculated delivery date and 78.4% delivered within 14 days. Similarly, calculations of the delivery date based on biparietal diameter measurements of less than 5.0 cm revealed that 70.5% delivered within 10 days of the calculated delivery date and 84.2% delivered within 14 days. Neither of these results were significantly better than those based on the date of the last menstrual period. Since the maximum fetal diameter measurements at 11 weeks of gestation and greater, upon which these calculations were made, are statistically
OBSTETRICS A N D GYNAECOLOCY
different from those of most other published series, the data obtained from maximum fetal diameters of rrreater than 33 mm were analysed. There were 32 such cases and a comparison was made of delivery date calculations from local data and from data published by Robinson and Flemming ( I 975). Local data calculations showed an accuracy of 65% delivering within I0 days of the calculated date and 81.2% delivering within 14 days whereas data from the London population showed accuracies of 40.6% and 56.2% respectively. Comparing the 10 day groups, 0.3>P>O.l, and in the 14 day groups 0.05 >P>0.01 . b
(b) Later Ultrusound Exutninution Amongst the same group of 137 patients, I86 ultrasonic examinations were used to calculate the delivery date. These included wherever possible one measurement between 8.0cm and 9.0cm and one measurement greater than but close to 9.0 crn. Tn the whole group calculations of the delivery date were within 10 days of the actual delivery date in 51.6% and within 14 days in 68.2%. In this group the use of the last menstrual period was significantly better (0.05>P>0.01) for calculating delivery date than ultrasonic measurements of the biparietal diameter. Analysis of the results from calculations based on biparietal diameters between 8.0 cm and 9.0 crn showed that 59.8% delivered within 10 days of the calculated delivery date and 75.9% delivered within 14 days. These figures did not show any significant advantage or disadvantage in calculatiny the delivery date from a biparietal diameter in this range compared with using the date of the last menstrual period. Finally, examination of the results of calculating the delivery date from biparietal diameters greater than 9.0cm gave figures of 44.4% and 61.6% respectively for delivery within 10 and 14 days. These figures show a significant disadvantage (0.01>P>0.001 in the 10 day group and 0.05>P >0.01 in t h e 14 day group) in using biparietal diameters in this range for calculating the delivery date. There was a significant difference (P9.0cm) ultrasonic examinations for predicting the delivery date.
Unsure of Dates Croup Of the 175 patients who laboured spontaneously, there were 21.7% who were unsure of dates, 39.0% who had been on hormonal contraception within 3 months of the last menstrual period, 16.0% who had experienced bleeding in early pregnancy, and 8.5% who had a prolonzed or irregular menstrual cycle. Examination of the delivery date data
G. B. DUFF
amongst the 102 who had some idea of the date of the last menstrual period revealed that 56.9% delivered within 1 0 days of the calculated delivery date and 66.6% delivered within 14 days. The corresponding figures from early ultrasonic measurements were 75.5% and 88.2% (P