In preterm pregnancies at risk for an adverse pregnancy outcome, antenatal tests are used to evaluate fetal well-being. The most widely used tests of fetal health are the biophysical profile (BPP),1 which assesses fetal breathing, movement, and tone; nonstress test; and amniotic fluid estimation and the modified biophysical profile,2 which uses only the nonstress test and amniotic fluid estimation (AFI). Both antenatal tests, if reassuring, are indicative of fetal well-being and have a low risk of an intrauterine fetal death within 3 days to 1 week of testing.3,4
Both antenatal tests use an estimation of the amniotic fluid volume as a fundamental assessment of “chronic” in utero stress. The modified BPP uses the AFI2 to estimate amniotic fluid volume, whereas the BPP uses the single deepest pocket (or presence of a 2 cm × 1 cm pocket of amniotic fluid) technique.5 Although both techniques have been employed extensively to assess the adequacy of amniotic fluid volume in research studies and clinical practice, the reliability of either test to predict outcome in at-risk pregnancies is uncertain.6–9 Although the techniques differ in the number of pregnancies that would be labeled as having oligohydramnios,10,11 the superiority of one technique over the other is unknown. The purpose of this investigation was to determine, in high-risk gestations, which technique of estimating amniotic fluid volume, the AFI or single deepest pocket, is the most accurate test to predict an adverse pregnancy outcome.
MATERIALS AND METHODS
Consecutive high-risk patients undergoing serial fetal surveillance were approached to participate in this randomized clinical trial. The inclusion criteria were gestations labeled as high risk because of maternal or fetal problems or both complicating pregnancy and being assessed with a weekly BPP. The patients were excluded if they had multifetal gestations or known fetal anomalies or refused study participation. This study was approved by the Institutional Review Board.
The AFI was obtained by dividing the abdomen into 4 quadrants, measuring the deepest pocket in each quadrant, and summing the AFI from each quadrant, as described by Phelan.2 Patients assigned to the single deepest pocket were evaluated according to Chamberlain.5 The vertical measurement of the single deepest pocket of amniotic fluid with a horizontal measurement of 1 cm and without fetal small parts or umbilical cord was measured in centimeters. Oligohydramnios, defined as AFI 5.0 cm or less or a single deepest pocket of less than 2 cm × 1 cm, was an indication for delivery if the gestational age was 34 weeks or more. Once a patient was assigned to AFI or single deepest pocket arm, all subsequent assessment of amniotic fluid used this technique. Clinicians managing the patients in the clinic were aware of the results of the BPP and whether the AFI or single deepest pocket was used to estimate the amniotic fluid volume. The amniotic fluid estimation (AFI or single deepest pocket) used for this study was the last volume estimated before delivery. In pregnancies complicated by diabetes or intrauterine growth restriction, the last amniotic fluid estimation was within 3 days of delivery and in all other pregnancies occurred within 7 days of delivery. The health care providers for labor and delivery were aware that these patients had participated in a study evaluating the BPP, including the estimation of amniotic fluid volume, and the last assessment which labeled the pregnancy as having normal or low fluid, but did not know the specific type of measurement (AFI or single deepest pocket) that was used.
Women with preterm premature rupture of membranes (PPROM) after 34 weeks were admitted and labor was induced. In women with PPROM before 34 weeks, they were managed conservatively and were induced at 34 weeks or before 34 weeks if they developed signs of chorioamnionitis. Women presenting a complication of pregnancy and oligohydramnios were induced if their gestational age was more than 34 weeks or admitted, administered corticosteroids, and delivered 24 hours after the last dose of steroids if the oligohydramnios persisted in those women less than 34 weeks gestation.
The primary endpoint in this trial was cesarean delivery for fetal distress. A sample size of 270 women per group was needed to attain 80% power to detect a difference in the rate of cesarean delivery for fetal intolerance of labor between treatment arms (11% using AFI ≤ 5 cm and 2% using a χ2 test at 5% significance level). For a 240-month period 540 women were randomly assigned to 1 of the 2 management regimens, using either AFI or single deepest pocket assessment. A randomization schedule was prepared in advance using a computer-generated number table with a card sealed in an opaque envelope that assigned patients to have the amniotic fluid assessed either with AFI or single deepest pocket.
Data were summarized using medians and interquartile ranges for continuous data and frequency distributions for categorical data. The primary endpoint and other categorical outcomes were investigated using χ2 tests. Supplementary analyses included logistic regression modeling to investigate factors simultaneously associated with each outcome considered (fetal distress in labor, cesarean delivery for fetal distress, admission to neonatal intensive care nursery, arterial pH < 7.1 and Apgar scores < 7), and the odds ratios (ORs) and 95% confidence intervals (CIs) were reported. To evaluate both modes of amniotic fluid monitoring, a new indicator variable was assigned to each pregnancy based on AFI or single deepest pocket assessment. Amniotic fluid volume was characterized as “normal” or “low,” and a comparison group consisting of patients who were monitored with single deepest pocket technique and where the single deepest pocket was more than 2 cm × 1 cm (normal volume) was used. In all logistic regression models BPP score components together with other patient characteristics were considered as candidate predictors for each outcome considered. Analysis was performed on an intent-to-treat basis. Statistical analysis was conducted using SPSS 11.0 statistical software (SPSS, Inc, Chicago, IL). Values of P < .05 were considered statistically significant.
Two hundred seventy-three women were monitored using the AFI and 264 women using single deepest pocket. Maternal characteristics were comparable between the monitoring groups (Table 1). As expected, pregnancy complications such as preterm premature rupture of membranes, hypertension, intrauterine growth restriction, and postterm pregnancies were the most common reasons for fetal health assessments. Diabetes was the most common preexisting condition (37/273, 14%, of women monitored with AFI and 28/264, 11%, of women monitored with single deepest pocket), and chronic hypertension was the second most common, with 17 (6%) compared with 14 (5%) of women monitored with AFI and single deepest pocket, respectively (9 women presented with both conditions). Other conditions antedating pregnancy included systemic lupus (1 case), human immunodeficiency virus–positive (1 case), and chronic renal disease (1 case).
Peripartum outcomes are summarized in Table 2. Statistically significant differences were found between the AFI-monitored and single deepest pocket–monitored groups because oligohydramnios was detected in 102 women (38%) in the AFI group compared with 46 women (17%) in the single deepest pocket group (P < .001, Table 2). The study groups were comparable with respect to the partial biophysical profile scores, those that combined fetal movement, tone, breathing, and non-stress test (P = .616), and became significantly different when the amniotic fluid assessment was also incorporated into the overall BPP score (P = .001).
A significantly higher rate of induction of labor for abnormal amniotic fluid indices was performed in the AFI monitored group (81 women, 30%, compared with 39 inductions, 15%, in the single deepest pocket monitored group, P < .001). A significant difference between rates of induction was found when inductions among women who presented with PPROM were considered (44 inductions, 66%, in the AFI group compared with 18 inductions, 29%, in the single deepest pocket monitored group, P < .001)
There were no differences between the incidence of variable (P = .808) or late decelerations (P = .095) and whether variability (P = .298) was absent or minimal. The overall rate of fetal intolerance of labor necessitating delivery by cesarean or vacuum and forceps was not significantly different between groups (P = .087), with 17% of the deliveries affected (45/273) in the AFI group and 11% of deliveries (30/264) in the single deepest pocket group. Mode of delivery was similar between the groups (P = .379), and the distributions of reasons for operative delivery were also alike (cesarean delivery, P = .228, or assisted vaginal deliveries, P = .808).
The rate of cesarean delivery for fetal distress was significantly different between the study groups (P = .014, 36/273, 13%, AFI compared with 18/264, 7%, single deepest pocket, Table 2). The tests were then stratified by AFI 5 or less, AFI more than 5, single deepest pocket absent or present and assessed by multivariate logistic regression analysis. Relative to patients with normal fluid using the single deepest pocket, women with oligohydramnios using the single deepest pocket were at similar risk for cesarean delivery for fetal intolerance of labor (P = .225; OR = 194, 95% CI 0.66–5.69), whereas women monitored with AFI were at increased risk of cesarean delivery (P = .028; OR = 2.57, 95% CI 1.11–5.97 for AFI < 5, and P = .024; OR = 2.41, 95% CI 1.12–5.20 for AFI > 5). Other significant predictors of cesarean delivery included older maternal age (P = .004; OR = 1.07, 95% CI 1.02–1.12), earlier gestational age at delivery (P = .002; OR = 0.90, 95% CI 0.84–0.96), administration of amnioinfusion (P = .038; OR = 2.45, 95% CI 1.05–5.88), and absent fetal tone in the BPP assessment (P = .021; OR = 7.59, 95% CI 1.25–42.54).
Similar results were obtained in subset analyses performed on pregnancies with normal fluid volume, low volume, and the combination of low and normal volumes. Relative to the women with normal fluid using the single deepest pocket, women with AFI more than 5 and AFI 5 or less were at significantly higher risk for undergoing cesarean delivery for fetal distress (P = .037, OR = 2.22, 95% CI 1.05–4.70, and P = .009, OR = 3.01, 95% CI 1.32–6.87, respectively), whereas the risk among women with oligohydramnios using the single deepest pocket did not reach statistical significance (P = .087, OR = 2.50, 95% CI 0.88–7.17). Three neonatal deaths occurred in this study. One occurred in the single deepest pocket group (PPROM with oligohydramnios using the single deepest pocket), 1 in the AFI group (diabetes with AFI 13.8), and the third one in the AFI group (PPROM with an AFI of 2.5). One of the neonates weighed 500 g (single deepest pocket group, 24 weeks gestation) and died of severe respiratory distress syndrome. The second infant weighed 850 grams (AFI group with PPROM at 27 weeks of gestation) and died secondary to prematurity, respiratory distress syndrome, and sepsis. The third pregnancy was complicated by type 1 diabetes (AFI 13.8, neonatal birth weight 3,250 g at 37 weeks of gestation), and the fetus developed fetal distress in labor (cord pH of 6.7 at delivery) and died soon after birth. There were no differences between the groups for any neonatal outcomes (Table 3). Umbilical artery rates of pH less than 7.1 were similar between the groups (P = .521 relative to single deepest pocket present). Fetal breathing movement was the only statistically significant predictor of pH less than 7.1 noted, and its absence was associated with an increased risk for low pH (P = .025;OR = 3.01, 95% CI 3.01–7.86). No differences in rates of pathologic acidosis were found between the groups (arterial pH < 7.0), with 8 (3%) newborns in the AFI group and 3 (1%) in the single deepest pocket group (P = .142).
The Apgar scores at 1 minute, adjusted for gestational age at delivery, were similar between the groups (P = .163), whereas Apgar scores at 5 minutes were different between the groups. When women with normal fluid using the single deepest pocket were compared with women with oligohydramnios using the single deepest pocket, they were at significantly increased risk of Apgar score less than 7 (P = .025; OR = 4.07, 95% CI 1.19–13.93), whereas women monitored with AFI were similar to those pregnancies with normal fluid using the single deepest pocket present (P = .484; OR = 1.57, 95% CI 0.44–5.59, for AFI > 5 and P = .340; OR = 1.77, 95% CI 0.55–5.72, for AFI ≤ 5). The study groups were also comparable with respect to admission to the special care nursery, tested univariately (P = .779) or with appropriate adjustments in the logistic regression analysis (P = .861).
The use of antenatal testing has become widespread in preterm pregnancies at a significant risk of fetal morbidity and mortality. An essential component of both tests is the estimation of amniotic fluid volume. Oligohydramnios, in a pregnancy without a fetal renal abnormality or genitourinary obstruction, is thought to represent “chronic” in utero stress. Two different methods of estimating the amniotic fluid are common in practice, so it is very important to establish which method is optimal for estimation of the amniotic fluid volume along with the other components of the BPP to identify the fetus at risk for an adverse outcome.
In this investigation, all of the pregnancies presented with clearly identified risk factors for an adverse outcome, so that the high number of women identified with oligohydramnios was expected in both groups. However, monitoring with the AFI resulted in a much greater number of pregnancies being classified as oligohydramnic (38%) compared with the single deepest pocket technique (17%). With a large number of women being labeled with oligohydramnios in the AFI group, does this improve our ability to recognize the at-risk pregnancies for an adverse intrapartum or neonatal outcome? Overall there was an increased number of women in the AFI group undergoing a cesarean delivery for fetal intolerance of labor compared with the single deepest pocket group. However, when the groups were separated into subgroups labeled as oligohydramnios by the AFI and single deepest pocket techniques, the cesarean delivery rate for fetal intolerance of labor was similar for the AFI and single deepest pocket groups. Astonishingly, there was no difference in the percentage of women delivered by cesarean for fetal intolerance of labor with oligohydramnios diagnosed by the AFI compared with women with a normal amniotic fluid volume by the AFI. Conversely, the number of women who had diagnosed normal fluid underwent a significantly greater number of cesarean deliveries for fetal intolerance of labor in the AFI group compared with the single deepest pocket group. In evaluating the intrapartum outcomes of the need for amnioinfusion, variable or late decelerations influencing delivery, and reasons for an operative delivery, there were no differences between groups.
Chauhan11 has reported that the knowledge of the amniotic fluid estimation by the health care provider in labor and delivery results in a greater number of cesarean deliveries for fetal intolerance in labor than if the estimation is unknown. In this investigation, with the results known to the provider, the cesarean delivery rates for fetal intolerance of labor were 36 of 102 with oligohydramnios estimated by the AFI and 18 of 46 in the single deepest pocket group. Despite the increased number of cesarean deliveries in the AFI compared with the single deepest pocket group, there were no differences in the neonatal outcomes of meconium, umbilical cord pH less than 7.1, 1-minute and 5-minute Apgar scores less than 7, or neonatal intensive care unit admissions between the 2 groups overall. However, a detailed comparison of outcomes stratified by group (AFI ≤ 5, AFI > 5, single deepest pocket present, and single deepest pocket absent) indicated that only the absence of single deepest pocket was predictive of low Apgar score at 5 minutes, and no differences were found for any other neonatal outcomes.
Higher numbers of pregnancies classified as having oligohydramnios by the AFI compared with single deepest pocket have previously been reported in postterm pregnancies. Alfirevic12 and colleagues observed that, although a greater number of the women were labeled as having low fluid, the perinatal outcomes were similar. They suggested that in pregnancies with a low risk of adverse outcomes, the diagnosis of low fluid may increase interventions and cause rather than prevent morbidity. The difference between the AFI and single deepest pocket in labeling a pregnancy as oligohydramniotic was demonstrated in an investigation in which 72% of the women with an AFI of 5 or less still had a single deepest pocket of more than 2 cm.10 The current study suggests that in high-risk pregnancies being serially monitored by the BPP, there is no consistent difference between the AFI and the single deepest pocket in their prediction of intrapartum complications or immediate neonatal outcomes. The AFI labels twice as many at-risk pregnancies as having low fluid volume as compared with the single deepest pocket technique, yet the percentage of pregnancies developing fetal intolerance of labor with oligohydramnios is similar between groups. Fewer pregnancies with amniotic fluid labeled as normal undergo cesarean deliveries for fetal distress in the single deepest pocket group compared with the AFI group. The single deepest pocket may be the better test for the clinical evaluation of amniotic fluid volume, because it seems to have poor sensitivity but good specificity whereas the AFI seems to have both poor sensitivity and poor specificity. This investigation implies that the single deepest pocket should remain as the methodology to estimate amniotic fluid volume along with the other components of the BPP and that the AFI should not be used in either BPP assessment.
1. Manning FA, Morrison I, Lange IR, Harman CR, Chamberlain PF. Fetal assessment based on fetal biophysical profile scoring: experience in 12,620 referred high-risk pregnancies. I. Perinatal mortality by frequency and etiology. Am J Obstet Gynecol 1985;151:343–50.
2. Phelan JP, Ahn MO, Smith CV, Rutherford SE, Anderson E. Amniotic fluid index measurements during pregnancy. J Reprod Med 1987;32:601–4.
3. Manning FA, Harman CR, Morrison I, Menticoglou SM, Lange IR, Johnson JM. Fetal assessment based on fetal biophysical profile scoring. IV. An analysis of perinatal morbidity and mortality. Am J Obstet Gynecol 1990;162:703–9.
4. Clark SL, Sabey P, Jolley K. Nonstress testing with acoustic stimulation and amniotic fluid volume assessment: 5973 tests without unexpected fetal death. Am J Obstet Gynecol 1989;160:694–7.
5. Chamberlain PF, Manning FA, Morrison I, Harman CR, Lange IR. Ultrasound evaluation of amniotic fluid volume. I. The relationship of marginal and decreased amniotic fluid volume to perinatal outcome. Am J Obstet Gynecol 1984;150:245–9.
6. Rutherford SE, Phelan JP, Smith CV, Jacobs N. The four-quadrant assessment of amniotic fluid volume: an adjunct to antepartum fetal heart rate testing. Obstet Gynecol 1987;70:353–6.
7. Casey BM, McIntire DD, Bloom SL, Lucas MJ, Santos R, Twickler DM, et al. Pregnancy outcomes after antepartum diagnosis of oligohydramnios at or beyond 34 weeks gestation. Am J Obstet Gynecol 2000;182:909–12.
8. Magann Ef, Chauhan SP, Kinsella MJ, McNamara MF, Whitworth NS, Morrison JC. Antenatal testing among 1001 patients at high risk: the role of ultrasonographic estimate of amniotic fluid volume. Am J Obstet Gynecol 1999;180:1330–6.
9. Chauhan SP, Sanderson M, Hendrix NW, Magann EF, Devoe LD. Perinatal outcome and amniotic fluid index in the antepartum and intrapartum periods: a meta-analysis. Am J Obstet Gynecol 1999;181:1473–8.
10. Magann EF, Kinsella MJ, Chauhan SP, McNamara MF, Gehring BW, Morrison JC. Does an amniotic fluid index </=5 cm necessitate delivery in high risk pregnancies? A case-control study. Am J Obstet Gynecol 1999;180:1354–9.
11. Chauhan SP, Washburne JF, Magann EF, Perry KG, Martin JN Jr., Morrison JC. A randomized study to assess the efficacy of the amniotic fluid index as a fetal admission test. Obstet Gynecol 1995;86:9–13.
12. Alfirevic Z, Luckas M, Walkinshaw SA, McFarlane M, Curran R. A randomized comparison between amniotic fluid index and maximum pool depth in the monitoring of post-term pregnancy. Br J Obstet Gynaecol 1997;104:207–11.