Determination of reference values for third trimester amniotic fluid index: a retrospective analysis of a large cohort of pregnancies with comparison to previous nomograms.

Purpose: To establish a new set of reference values for third-trimester amniotic fluid index (AFI) and compare them to other previously published normograms.Methods: A retrospective cross-sectional cohort analysis of all singleton sonographic evaluations >22 gestational weeks in one university affiliated medical center between 2013 and 2017. Pregnancies complicated by rupture of membranes, major anomalies/chromosomal abnormalities were excluded. One evaluation per patient per pregnancy was randomly selected. Reference values were constructed using a best-fit regression model for estimation of mean and standard deviation at each gestational age after normalization of variables and compared with previously published norms.Results: A total of 7037 ultrasound evaluations entered the analysis. Correlation between AFI and gestational age was best represented by a first-degree polynomial equation. AFI decreased gradually from 16.4 at 22 weeks to 13.3 at 40 weeks (cm, median). The standard deviation increased with gestational age with AFI ranging from 12.9-20.2 at 22 weeks and 4.7-26.2 at 40 weeks (cm, 2.5-97.5 percentile). Compared to other curves, our reference values demonstrated a higher median AFI throughout all gestation.Conclusions: Reference values for the third trimester AFI were established. Curves should be correlated with perinatal outcome prior to wide clinical implementation.