203: A contemporary amniotic fluid volume standard for the United States: the NICHD fetal growth studies-singletons

Abstract Background Amniotic fluid is essential to normal fetal development and is estimated clinically with ultrasound to identify pregnancies at risk for poor perinatal outcome, Objectives Our goal was to develop a United States standard for amniotic fluid volume estimated by the amniotic fluid index and single deepest pocket. Study Design We performed a planned secondary analysis of a multicenter observational study of 2334 low-risk women with normal singleton gestations from one of 4 self-reported racial-ethnic groups. Eligible women had confirmed first-trimester dating criteria with health status, lifestyles and medical and obstetric histories associated with normal fetal growth. Consenting women underwent serial (up to 5) sonographic evaluations of amniotic fluid between 15 and 40 weeks’ gestation after being randomly assigned to one of 4 gestational age observation schedules. Twelve U.S. perinatal centers participated, and all sonograms were performed by credentialed sonographers using identical, high-resolution equipment; caregivers were unaware of results but were notified for oligohydramnios. Women (N=597) subsequently found to have clinically significant antepartum complications were excluded. Racial-ethnic-specific nomograms for amniotic fluid index and single deepest pocket across gestation were developed using linear mixed models with cubic splines, while racial-ethnic differences were evaluated both with global and between-group tests. Median, 3 rd , 5 th , 10 th , 90 th , 95 th and 97 th percentile values were also estimated. We further considered the possible confounding effects of selected maternal characteristics and the estimated fetal weight at each sonogram. Results A total of 1719 pregnant women met inclusion criteria and had available data. These included 480 non-Hispanic white, 418 non-Hispanic black, 485 Hispanic and 336 Asian women. Both the amniotic fluid index and the single deepest pocket varied across gestation with maximal values at 26 and 33 weeks respectively. Statistically significant differences were observed by maternal race-ethnicity. The between-group differences observed at 17-22 weeks’ and 35-40 weeks’ gestation remained statistically significant after adjusting for maternal characteristics and estimated fetal weight. These between-group racial-ethnic differences were most prominent after 35 weeks’ gestation and at the extremes of dispersion (3 rd and 97 th percentiles). All 3 rd and 97 th percentile amniotic fluid index values were within the range of commonly used cutoffs to define oligohydramnios (≤ 5 cm) and polyhydramnios (≥ 25 cm). However, 3 rd percentile values ranged between 5.9 cm at 40 weeks and 10.1 cm at 25-27 weeks, while the 97 th percentile values ranged between 24.8 cm at 38 weeks and 15.7 cm at 15 weeks. Conclusions Sonographic amniotic fluid volume estimates vary by racial-ethnic group, but the absolute differences appear to be small and may not be clinically significant. Selected maternal characteristics and estimated fetal weight did not affect the racial-ethnic differences. Between-group differences are maximal after 35 weeks and at the extremes of the upper and lower dispersion estimates. Given the observed variability in extreme (3 rd and 97 th percentile) dispersion values over gestation, use of single cutoffs to define out-of-range measurements may not be clinically appropriate. These data might form a contemporary United States standard for amniotic fluid estimation using the amniotic fluid index and the single deepest pocket. ClinicalTrials.gov Identifier for parent study : NCT00912132