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

      Objective

      To develop a contemporary amniotic fluid volume (AFV) standard for the U.S. and assess racial/ethnic differences.

      Study Design

      1,719 low-risk gravidas of 4 self-reported racial/ethnic groups, carrying singletons, had serial (q 4 wk) sonograms at 12 U.S. sites, 2010-2013. Low-risk was defined: age 18-40 yr, BMI 19-29.9 kg/m2, healthy lifestyle, normal obstetric history and uncomplicated pregnancy. Protocol trained, credentialed sonographers measured the 4-quadrant amniotic fluid index (AFI) at each scan, 15-40 wks’ gestation (GA), as determined by a certain LMP, confirmed by first-trimester biometry. Women were randomly assigned to 1 of 4 sonography groups to ensure that each GA week was sampled. AFI trajectories were estimated using linear mixed models with cubic splines as fixed effects and cubic terms as random effects. AFI values were compared by race/ethnicity after adjusting for age, height, weight, parity, employment, marital status, insurance, income and education. We further adjusted for estimated fetal weight (EFW).

      Results

      480 non-Hispanic white (NHW), 418 non-Hispanic black (NHB), 485 Hispanics and 366 Asians underwent 7,177 scans. The population median AFI & percentiles are shown in the figure. Median AFI peaked at 27 wks (15.4 cm) falling to 12.9 cm by 40 wks. At 40 wks the population 2.5th %ile was 2.6 cm versus 97.5th %ile, 23.1 cm (table). Significant differences were observed among racial/ethnic groups from 18-21 and 26-40 wks (p<0.001): at 40 wks NHW had the largest and NHB the smallest median (delta=1.5cm; p<0.001). Racial/ethnic differences persisted after adjusting for EFW.

      Conclusion

      We present a contemporary AFV standard for the U.S. based on a large, low-risk multi-racial/ethnic population, evaluated serially by credentialed sonographers at 12 U.S. centers. Statistically significant racial/ethnic differences were observed. This information may help inform future clinical studies of amniotic fluid volume assessment and obstetric management.
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