European Journal of Obstetrics & Gynecology and Reproductive Biology, 46 (1992) l-5 0 1992 Elsevier
SciencePublishersB.V. All rights reserved 0028-2243/92/$05.00
EUROBS 01385
Correlation of subjective assessment of amniotic fluid with amniotic fluid index Keith Williams,
Bernd K. Wittmann
and Jerome
Dansereau
The Universityof British Columbia, Department of Obstetrics & Gynaecology, Division of Maternal Fetal Medicine, Department of Obstetrics and Gynaecology, Grace Maternity Hospital, Vancouver, BC, Canada
Accepted for publication 22 April 1992
Summary We assessed the correlation between abnormal amniotic fluid volumes as defined by the two techniques of (1) subjective evaluation and (2) the amniotic fluid index. Ultrasound evaluation of amniotic fluid volume was conducted on 420 pregnant women with known gestational age greater than twenty weeks but less than 42 weeks. Amniotic fluid was evaluated subjectively and placed into one of three categories: normal, oligohydramnios or polyhydramnios. After fetal biometry was performed, the amniotic fluid volume was assessed semi-quantitatively by the amniotic fluid index technique and assigned to similar categories. We analyzed the data with 2 X 2 contingency tables, using amniotic fluid index as the ‘gold standard test’. Our study demonstrates that there was moderate agreement (K.5) between both amniotic fluid techniques in the identification of oligohydramnios. However, agreement between the techniques was poor for the identification of polyhydramnios (~.16). Amniotic fluid assessment; Amniotic fluid index; subjective assessment
Introduction Abnormalities of amniotic fluid volume have been shown to be associated with complications during pregnancy such as diabetes, rhesus isoimmunization, and intrauterine growth retardation (IUGR). [l-51. Therefore, accurate assessment of amniotic fluid is an important aspect of the evaluation of fetal well-being. Although ultrasound provides a safe, non-invasive technique for measurement of amniotic fluid
Correspondence
to: Dr. Keith Williams, Assistant Professor Department of Obstetrics & Gynaecology, UBC Room IT2A, Grace Maternity Hospital 4490 Oak Street, Vancouver, BC V6H 3V5, Canada.
volume, the current sonographic techniques are indirect measurements, and provide only estimates of amniotic fluid volume. Several sonographic techniques designed to evaluate amniotic fluid volume have been reported in the literature, including subjective assessment 121,maximum vertical pocket depth (MVP) [ll, and amniotic fluid index (AFI) [6]. Of these measures, amniotic fluid index is currently the ‘gold standard’ since it has been shown to be reproducible [12] and one report by Moore et al. [7] found a correlation coefficient of 0.88 between amniotic fluid index and actual amniotic fluid volumes in sheep, thereby confirming the validity of the measure. In our department, subjective evaluation of amniotic fluid is performed according to the cri-
2
teria of Philipson et al. [2] for defining oligohydramnios. Halperin et al. [4] showed that subjectively defined amniotic fluid categories have good reliability. The purpose of the present study is to compare the ability of subjective evaluation to detect abnormal amniotic fluid volumes (oliogohydramnios and polyhydramnios) identified by the amniotic fluid index (API) technique. Materials and Methods Ultrasound evaluation of amniotic fluid volume was conducted on 420 pregnant women referred to the Ultrasound Department of the Grace Maternity Hospital for a variety of indications. Inclusion criteria were: known gestational age greater than 20 weeks but less than 42 weeks, either by confirmed last menstrual period of early first trimester dating ultrasound scan. Excluded were: known twin gestations and premature rupture of membranes. Informed consent was obtained from all patients. All patients were scanned using a 3.5 mHz probe real time linear scanner. Transverse cuts of the uterine cavity were made at several levels above and below the umbilicus and a hard ultrasound copy was made. All films were reviewed by one investigator with no knowledge of the patient’s clinical condition. Decreased amniotic fluid volume was thus assessed subjectively using criteria previously described by Philipson et al. [2]; overall sense of crowding of the fetus, an obvious lack of amniotic fluid, and/or inability to identify any significant pockets of fluid in any sector of the uterus. Excess fluid is defined subjectively when there is obvious excessive fluid and when transverse scans done at various levels of the uterine cavity identify a fluid pocket in which one can comfortably place a cross-section of the fetal trunk [16]. The investigator then assigned a subjective score of normal, oligohydramnios or polyhydramnios. After obtaining the ultrasound films for subjective evaluation, fetal biometry was performed. The API was assessed last, using the technique described by Phelan et al. [6] in which the maternal abdomen is divided into four quadrants. The
linea nigra was utilized to divide the abdomen into the right and left halves and the umbilicus was used to separate upper and lower halves. With the ‘transducer held perpendicular to the floor, the largest amniotic fluid pocket in each quadrant was identified. The vertical diameter of the largest pocket in each quadrant was measured and a hard ultrasound copy made. All scans were reviewed by the investigator blinded to the patient’s clinical condition. The depths for the four quadrants were then added by this investigator to obtain the API in millimeters. The following modifications were introduced to the technique: (a) all patients were placed in slight left lateral tilt to displace the uterus and prevent supine hypotension syndrome. (b) in all patients at gestational age less than 26 weeks the uterus was divided into the upper and lower halves by a line midway between the top of the fundus and symphysis pubis. Oligohydramnios was defined as an API Q 5th percentile for gestational age and polyhydramnios was defined as an AFI > 95th percentile for gestational age using normal values produced by Moore et al. [8]. Amniotic fluid volume categories obtained by subjective assessment were compared with those obtained with the API technique. Statistical analysis was performed by developing 2 x 2 contingency tables for the amniotic fluid volume categories and calculating the sensitivity, specificity, positive and negative predictive values and correlation using the K statistic with the 95th confidence intervals. Results One hundred of the 420 subjects (24%) were referred for assessment of fetal growth and the others for diverse reasons detailed in Table I. Similar numbers of patients were identified as having abnormally decreased amniotic fluid, 39 (9%) patients by the API technique as compared to 45 (11%) being identified by the subjective technique (Table II). Whereas 41 (10%) patients were identified as having excessive amniotic fluid this contrasted with only 10 patients (2%), who
3 TABLE I
TABLE II
Indications for referral for ultrasound
Number and percent of subjects diagnosed with oligohydramnios, normal amniotic fluid volume and polyhydramnios by amniotic fluid index and by subjective assessment (n = 420)
Indication
n
Assess growth Diabetes Hypertension Post term Suspected IUGR Follow-up ultrasound of previous normal fetal growth
100 84 46 45 38
24 20 11 11 9
-107
25 _
Total
420
100
%
Oligohydramnios Normal Polyhydramnios
were identified with polyhydramnios as defined by an AFI greater than the 95th centile. Overall agreement of the techniques in the identification of abnormal amniotic fluid volumes in the three categories of oligohydramnios, normal and polyhydramnios was poor (K 0.37). However there was moderate agreement (K 0.5) between both amniotic techniques in the identification of oligohydramnios (Table III>. Agreement between the techniques was poor for the identification of polyhydramnios (K 0.16). The sensitivity specificity, the positive predictive values as described in (Table IV> portray the subjective assessment technique as a very specific technique in identifying abnormal amniotic fluid volumes as defined by the AFI technique (specificity 95% for oligohydramnios and 90% for polyhy-
Amniotic fluid index
Subjective
n
%
n
%
39 371 10
9 88 2
45 334 41
11 80 10
dramnios). However, the technique shows only moderate positive predictive value in identify oligohydramnios (51%) which worsened to 12% in the identification of polyhydramnios. Discussion Because of the irregularity of the uterine cavity and the changing position of the fetus, accurate of amniotic fluid volume would require invasive techniques of amniocentesis and dye injection [9-111. Ultrasound imaging has allowed for the assessment of amniotic fluid volume in a non-invasive fashion. A number of techniques have been developed in an attempt to quantify amniotic fluid volume. These include the Maximum Vertical Pocket (MVP), the Amniotic Fluid Index and subjective assessment but all suffer the major disadvantage of being semi-quantitative.
TABLE III 3 X 3 contingency table of amniotic fluid evaluation subjective assessment vs. amniotic fluid index Amniotic fluid index Subjective
AFI I 5th centile oligohydramnios
5 < AFI zc 95th centile normal
Oligohydramnios Normal Polyhydramnios
23 16 0
22 313 36
0 5 5
45 334 41
Total
39
371
10
420
AFI > 95th centile polyhydramnios
K score with (95% centile confidence intervals) K = 0.37 (0.26-0.47) three categories oligohydramnios, normal, polyhydramnios. K = 0.50 (0.35-0.62) two categories oligohydramnios vs normal using the AFI as the gold standard. K = 0.16 (0.02-0.311 two categories polyhydramnios vs normal using the AFI as the gold standard.
Total
4 TABLE IV Accuracy of the subjective assessment technique in identifying oligohydramnios fluid index Subjective assessment Oligohydramnios Polyhydramnios
and polyhydramnios as defined by the amniotic
Sensitivity (%I
Specificity (%,)
Positive predictive value (%I
Negative predictive value (%I
Prevalence (a)
59
95
51
96
9.5
50
90
12
99
2.6
The AFI has been recommended for use as the ‘gold standard’ of all these techniques because it is reproducible [13,14]. Its validity was assessed by Moore et al. [7], who compared the ability of the API to predict known amniotic fluid volumes in the sheep and showed a linear relationship to actual amniotic fluid volumes with 88% accuracy to quantitative amniotic fluid volume. Because of this known good reproducibility and validity we used the AFI as the ‘gold standard’ for defining amniotic fluid volumes. As amniotic fluid volumes are expressed numerically by the AFI technique this allows ease of comparison of small amniotic fluid volume changes which is not achieved by the subjective technique. Previously Halperin et al. [12], in evaluating the subjective technique, demonstrated good to excellent intraobserver and interobserver reliability assessment with Kappas of 0.94 and 0.88, respectively, for two experienced observers, All our subjective assessments were performed by one ultrasound physician (B.K.W.) who has over 20 years experience in interpretating ultrasound. We compared the subjective evaluation with the AFI as the ‘gold standard’ therefore to asses the validity of this subjective eyeballing technique. Previously when the AFI technique has been used as the gold standard and compared against the MVP technique of amniotic fluid assessment the correlation was poor [151. Our study demonstrates that if one accepts API as a ‘gold standard’, the subjective evaluation of amniotic fluid volume shows moderate ability in identifying oligohydramnios which worsens in attempting to predict polyhydramnios.
Acknowledgements This study was funded by the B.C. Medical Services Foundation. Grant number #88-38. We would like to thank the staff of the Grace Hospital Ultrasound Department. References Chamberlain PF, Manning FA, Morrison I, Harman CR, Lange TR. Ultrasound evaluation of amniotic fluid volume I. The relationship of marginal and decreased amniotic fluid volumes to perinatal outcome. Am J Obstet Gynecol 1984;150:245-249. Philipson EH, Sokol RJ, Williams T. Oligohydramnios: clinical associations and predictive value for intrauterine growth retardation. Am J Obstet Gynecol 1983;146:217278. Rutherford SE, Phelan JP, Smith CV, Jacobs N. The four quadrant assessment of amniotic fluid: an adjunct to antepartum fetal heart rate testing. Am J Obstet Gynecol 1987;80:353-386. Shymoys SM, Sirkim M, Dery C, Monheit AG, Baker DA. Amnioti fluid index: an appropriate predictor of perinatal outcome. Am J Perinat 1990$266-269. Van Regemorter N, Vamos E, DeFleur V, El Khazen N, Jeanty Ph, Levi S, Avni F, Foulton W, Liebaers I, Rodesch F. Pathological pregnancies: results of amniotic fluid studies and fetal outcome. Acta Obstet Gynecol Stand 1986;65:27-32. Phelan JP, Smith CV, Brossard P, Small M. Amniotic fluid volume assessment with the four quadrant techniques at 36-42 weeks gestation. J Reprod Med 1987;32:540-542. Moore TR, Brace RA. Amniotic fluid index (AFI) in the term ovine pregnancy: a predictable relationship between AFI and amniotic fluid volume. In: Proceedings of the 35th annual meeting of the Society for Gynecologic Investigation, Baltimore, MD, March 17-20. Baltimore Society for Gynecologic Investigation, 1988.
5
8 MooreTR, GayleJE. The amnioticfluidindexin normal human pregnancy. Am J Obstet Gynecol 1990;162:11681173. 9 Charles D, Jacoby H, Burgess F. Amniotic fluid volumes in the second half of pregnancy. Am J Obstet Gynecol 1965;93:1042-1047. 10 Brace RA, Wolf EJ. Characterization of normal gestational changes in amniotic fluid volume. Am J Obstet Gynecol 1989;161:382-338. 11 Haswell GL, Morris JA. Amniotic fluid volume studies. Am J Obstet Gynecel 1973;42:725-732. 12 Halperin ME, Fong KW, Zalev A.lt Goldsmith CH. Reliability of amniotic fluid volume estimation from ultrasonograms: Interobserver and intraobsetver variation before and after establishment of criteria. Am J Obstet Gynecol 1985;153:264-267.
13 RutherfordSE,SmithCV,PhelanJP, Kawakami K, Ahn MD. The four quadrantassessmentof amnioticfluid: interobserver and intraobserver variation. J Reprod Med 1987;32:587-589. 14 Williams KP, Wittman BK. Amniotic fluid index reproducibility of the technique. In: Proceedings of the 47th annual meeting of the Society of Obstetricians and Gynaecologists of Canada, Toronto, Ontario, Canada. June ll15, 1991. 15 Moore TR. Superiority of the four-quadrant sum over the single deepest pocket technique in ultrasonographic identification of abnormal amniotic fluid volumes. Am J Obstet Gynecol 1990;163:762-767. 16 Hansmann M, Hackelijre BJ, Staudach A. Ultrasound Diagnosis in Obstetrics and Gynecology. Springer-Verlag, Berlin, Heidelberg. 1986; Chapter 8:190.
SciencePublishersB.V. All rights reserved 0028-2243/92/$05.00
EUROBS 01385
Correlation of subjective assessment of amniotic fluid with amniotic fluid index Keith Williams,
Bernd K. Wittmann
and Jerome
Dansereau
The Universityof British Columbia, Department of Obstetrics & Gynaecology, Division of Maternal Fetal Medicine, Department of Obstetrics and Gynaecology, Grace Maternity Hospital, Vancouver, BC, Canada
Accepted for publication 22 April 1992
Summary We assessed the correlation between abnormal amniotic fluid volumes as defined by the two techniques of (1) subjective evaluation and (2) the amniotic fluid index. Ultrasound evaluation of amniotic fluid volume was conducted on 420 pregnant women with known gestational age greater than twenty weeks but less than 42 weeks. Amniotic fluid was evaluated subjectively and placed into one of three categories: normal, oligohydramnios or polyhydramnios. After fetal biometry was performed, the amniotic fluid volume was assessed semi-quantitatively by the amniotic fluid index technique and assigned to similar categories. We analyzed the data with 2 X 2 contingency tables, using amniotic fluid index as the ‘gold standard test’. Our study demonstrates that there was moderate agreement (K.5) between both amniotic fluid techniques in the identification of oligohydramnios. However, agreement between the techniques was poor for the identification of polyhydramnios (~.16). Amniotic fluid assessment; Amniotic fluid index; subjective assessment
Introduction Abnormalities of amniotic fluid volume have been shown to be associated with complications during pregnancy such as diabetes, rhesus isoimmunization, and intrauterine growth retardation (IUGR). [l-51. Therefore, accurate assessment of amniotic fluid is an important aspect of the evaluation of fetal well-being. Although ultrasound provides a safe, non-invasive technique for measurement of amniotic fluid
Correspondence
to: Dr. Keith Williams, Assistant Professor Department of Obstetrics & Gynaecology, UBC Room IT2A, Grace Maternity Hospital 4490 Oak Street, Vancouver, BC V6H 3V5, Canada.
volume, the current sonographic techniques are indirect measurements, and provide only estimates of amniotic fluid volume. Several sonographic techniques designed to evaluate amniotic fluid volume have been reported in the literature, including subjective assessment 121,maximum vertical pocket depth (MVP) [ll, and amniotic fluid index (AFI) [6]. Of these measures, amniotic fluid index is currently the ‘gold standard’ since it has been shown to be reproducible [12] and one report by Moore et al. [7] found a correlation coefficient of 0.88 between amniotic fluid index and actual amniotic fluid volumes in sheep, thereby confirming the validity of the measure. In our department, subjective evaluation of amniotic fluid is performed according to the cri-
2
teria of Philipson et al. [2] for defining oligohydramnios. Halperin et al. [4] showed that subjectively defined amniotic fluid categories have good reliability. The purpose of the present study is to compare the ability of subjective evaluation to detect abnormal amniotic fluid volumes (oliogohydramnios and polyhydramnios) identified by the amniotic fluid index (API) technique. Materials and Methods Ultrasound evaluation of amniotic fluid volume was conducted on 420 pregnant women referred to the Ultrasound Department of the Grace Maternity Hospital for a variety of indications. Inclusion criteria were: known gestational age greater than 20 weeks but less than 42 weeks, either by confirmed last menstrual period of early first trimester dating ultrasound scan. Excluded were: known twin gestations and premature rupture of membranes. Informed consent was obtained from all patients. All patients were scanned using a 3.5 mHz probe real time linear scanner. Transverse cuts of the uterine cavity were made at several levels above and below the umbilicus and a hard ultrasound copy was made. All films were reviewed by one investigator with no knowledge of the patient’s clinical condition. Decreased amniotic fluid volume was thus assessed subjectively using criteria previously described by Philipson et al. [2]; overall sense of crowding of the fetus, an obvious lack of amniotic fluid, and/or inability to identify any significant pockets of fluid in any sector of the uterus. Excess fluid is defined subjectively when there is obvious excessive fluid and when transverse scans done at various levels of the uterine cavity identify a fluid pocket in which one can comfortably place a cross-section of the fetal trunk [16]. The investigator then assigned a subjective score of normal, oligohydramnios or polyhydramnios. After obtaining the ultrasound films for subjective evaluation, fetal biometry was performed. The API was assessed last, using the technique described by Phelan et al. [6] in which the maternal abdomen is divided into four quadrants. The
linea nigra was utilized to divide the abdomen into the right and left halves and the umbilicus was used to separate upper and lower halves. With the ‘transducer held perpendicular to the floor, the largest amniotic fluid pocket in each quadrant was identified. The vertical diameter of the largest pocket in each quadrant was measured and a hard ultrasound copy made. All scans were reviewed by the investigator blinded to the patient’s clinical condition. The depths for the four quadrants were then added by this investigator to obtain the API in millimeters. The following modifications were introduced to the technique: (a) all patients were placed in slight left lateral tilt to displace the uterus and prevent supine hypotension syndrome. (b) in all patients at gestational age less than 26 weeks the uterus was divided into the upper and lower halves by a line midway between the top of the fundus and symphysis pubis. Oligohydramnios was defined as an API Q 5th percentile for gestational age and polyhydramnios was defined as an AFI > 95th percentile for gestational age using normal values produced by Moore et al. [8]. Amniotic fluid volume categories obtained by subjective assessment were compared with those obtained with the API technique. Statistical analysis was performed by developing 2 x 2 contingency tables for the amniotic fluid volume categories and calculating the sensitivity, specificity, positive and negative predictive values and correlation using the K statistic with the 95th confidence intervals. Results One hundred of the 420 subjects (24%) were referred for assessment of fetal growth and the others for diverse reasons detailed in Table I. Similar numbers of patients were identified as having abnormally decreased amniotic fluid, 39 (9%) patients by the API technique as compared to 45 (11%) being identified by the subjective technique (Table II). Whereas 41 (10%) patients were identified as having excessive amniotic fluid this contrasted with only 10 patients (2%), who
3 TABLE I
TABLE II
Indications for referral for ultrasound
Number and percent of subjects diagnosed with oligohydramnios, normal amniotic fluid volume and polyhydramnios by amniotic fluid index and by subjective assessment (n = 420)
Indication
n
Assess growth Diabetes Hypertension Post term Suspected IUGR Follow-up ultrasound of previous normal fetal growth
100 84 46 45 38
24 20 11 11 9
-107
25 _
Total
420
100
%
Oligohydramnios Normal Polyhydramnios
were identified with polyhydramnios as defined by an AFI greater than the 95th centile. Overall agreement of the techniques in the identification of abnormal amniotic fluid volumes in the three categories of oligohydramnios, normal and polyhydramnios was poor (K 0.37). However there was moderate agreement (K 0.5) between both amniotic techniques in the identification of oligohydramnios (Table III>. Agreement between the techniques was poor for the identification of polyhydramnios (K 0.16). The sensitivity specificity, the positive predictive values as described in (Table IV> portray the subjective assessment technique as a very specific technique in identifying abnormal amniotic fluid volumes as defined by the AFI technique (specificity 95% for oligohydramnios and 90% for polyhy-
Amniotic fluid index
Subjective
n
%
n
%
39 371 10
9 88 2
45 334 41
11 80 10
dramnios). However, the technique shows only moderate positive predictive value in identify oligohydramnios (51%) which worsened to 12% in the identification of polyhydramnios. Discussion Because of the irregularity of the uterine cavity and the changing position of the fetus, accurate of amniotic fluid volume would require invasive techniques of amniocentesis and dye injection [9-111. Ultrasound imaging has allowed for the assessment of amniotic fluid volume in a non-invasive fashion. A number of techniques have been developed in an attempt to quantify amniotic fluid volume. These include the Maximum Vertical Pocket (MVP), the Amniotic Fluid Index and subjective assessment but all suffer the major disadvantage of being semi-quantitative.
TABLE III 3 X 3 contingency table of amniotic fluid evaluation subjective assessment vs. amniotic fluid index Amniotic fluid index Subjective
AFI I 5th centile oligohydramnios
5 < AFI zc 95th centile normal
Oligohydramnios Normal Polyhydramnios
23 16 0
22 313 36
0 5 5
45 334 41
Total
39
371
10
420
AFI > 95th centile polyhydramnios
K score with (95% centile confidence intervals) K = 0.37 (0.26-0.47) three categories oligohydramnios, normal, polyhydramnios. K = 0.50 (0.35-0.62) two categories oligohydramnios vs normal using the AFI as the gold standard. K = 0.16 (0.02-0.311 two categories polyhydramnios vs normal using the AFI as the gold standard.
Total
4 TABLE IV Accuracy of the subjective assessment technique in identifying oligohydramnios fluid index Subjective assessment Oligohydramnios Polyhydramnios
and polyhydramnios as defined by the amniotic
Sensitivity (%I
Specificity (%,)
Positive predictive value (%I
Negative predictive value (%I
Prevalence (a)
59
95
51
96
9.5
50
90
12
99
2.6
The AFI has been recommended for use as the ‘gold standard’ of all these techniques because it is reproducible [13,14]. Its validity was assessed by Moore et al. [7], who compared the ability of the API to predict known amniotic fluid volumes in the sheep and showed a linear relationship to actual amniotic fluid volumes with 88% accuracy to quantitative amniotic fluid volume. Because of this known good reproducibility and validity we used the AFI as the ‘gold standard’ for defining amniotic fluid volumes. As amniotic fluid volumes are expressed numerically by the AFI technique this allows ease of comparison of small amniotic fluid volume changes which is not achieved by the subjective technique. Previously Halperin et al. [12], in evaluating the subjective technique, demonstrated good to excellent intraobserver and interobserver reliability assessment with Kappas of 0.94 and 0.88, respectively, for two experienced observers, All our subjective assessments were performed by one ultrasound physician (B.K.W.) who has over 20 years experience in interpretating ultrasound. We compared the subjective evaluation with the AFI as the ‘gold standard’ therefore to asses the validity of this subjective eyeballing technique. Previously when the AFI technique has been used as the gold standard and compared against the MVP technique of amniotic fluid assessment the correlation was poor [151. Our study demonstrates that if one accepts API as a ‘gold standard’, the subjective evaluation of amniotic fluid volume shows moderate ability in identifying oligohydramnios which worsens in attempting to predict polyhydramnios.
Acknowledgements This study was funded by the B.C. Medical Services Foundation. Grant number #88-38. We would like to thank the staff of the Grace Hospital Ultrasound Department. References Chamberlain PF, Manning FA, Morrison I, Harman CR, Lange TR. Ultrasound evaluation of amniotic fluid volume I. The relationship of marginal and decreased amniotic fluid volumes to perinatal outcome. Am J Obstet Gynecol 1984;150:245-249. Philipson EH, Sokol RJ, Williams T. Oligohydramnios: clinical associations and predictive value for intrauterine growth retardation. Am J Obstet Gynecol 1983;146:217278. Rutherford SE, Phelan JP, Smith CV, Jacobs N. The four quadrant assessment of amniotic fluid: an adjunct to antepartum fetal heart rate testing. Am J Obstet Gynecol 1987;80:353-386. Shymoys SM, Sirkim M, Dery C, Monheit AG, Baker DA. Amnioti fluid index: an appropriate predictor of perinatal outcome. Am J Perinat 1990$266-269. Van Regemorter N, Vamos E, DeFleur V, El Khazen N, Jeanty Ph, Levi S, Avni F, Foulton W, Liebaers I, Rodesch F. Pathological pregnancies: results of amniotic fluid studies and fetal outcome. Acta Obstet Gynecol Stand 1986;65:27-32. Phelan JP, Smith CV, Brossard P, Small M. Amniotic fluid volume assessment with the four quadrant techniques at 36-42 weeks gestation. J Reprod Med 1987;32:540-542. Moore TR, Brace RA. Amniotic fluid index (AFI) in the term ovine pregnancy: a predictable relationship between AFI and amniotic fluid volume. In: Proceedings of the 35th annual meeting of the Society for Gynecologic Investigation, Baltimore, MD, March 17-20. Baltimore Society for Gynecologic Investigation, 1988.
5
8 MooreTR, GayleJE. The amnioticfluidindexin normal human pregnancy. Am J Obstet Gynecol 1990;162:11681173. 9 Charles D, Jacoby H, Burgess F. Amniotic fluid volumes in the second half of pregnancy. Am J Obstet Gynecol 1965;93:1042-1047. 10 Brace RA, Wolf EJ. Characterization of normal gestational changes in amniotic fluid volume. Am J Obstet Gynecol 1989;161:382-338. 11 Haswell GL, Morris JA. Amniotic fluid volume studies. Am J Obstet Gynecel 1973;42:725-732. 12 Halperin ME, Fong KW, Zalev A.lt Goldsmith CH. Reliability of amniotic fluid volume estimation from ultrasonograms: Interobserver and intraobsetver variation before and after establishment of criteria. Am J Obstet Gynecol 1985;153:264-267.
13 RutherfordSE,SmithCV,PhelanJP, Kawakami K, Ahn MD. The four quadrantassessmentof amnioticfluid: interobserver and intraobserver variation. J Reprod Med 1987;32:587-589. 14 Williams KP, Wittman BK. Amniotic fluid index reproducibility of the technique. In: Proceedings of the 47th annual meeting of the Society of Obstetricians and Gynaecologists of Canada, Toronto, Ontario, Canada. June ll15, 1991. 15 Moore TR. Superiority of the four-quadrant sum over the single deepest pocket technique in ultrasonographic identification of abnormal amniotic fluid volumes. Am J Obstet Gynecol 1990;163:762-767. 16 Hansmann M, Hackelijre BJ, Staudach A. Ultrasound Diagnosis in Obstetrics and Gynecology. Springer-Verlag, Berlin, Heidelberg. 1986; Chapter 8:190.
