Longitudinal assessment of amniotic fluid index

Abstract

Objective: To derive a longitudinal gestational reference range for amniotic fluid index and to compare it with the established cross-sectional reference range.

Design: Longitudinal prospective study.

Setting: Liverpool Maternity Hospital and Mill Road Maternity Hospital, both teaching hospitals in central Liverpool.

Subjects: One hundred and fifteen pregnant women, in whom the expected date by the last menstrual period and ultrasound scan at 18 to 19 weeks were in agreement by seven days, were recruited. They were divided into two groups and scanned at four weekly intervals as follows: Group A: scanned at 20, 24, 28, 32, 36, 40; and Group B: scanned at 22, 26, 30, 34, 38, 42.

Results: Amniotic fluid index varies with gestation, rising from early gestation to peak at 30 weeks and then falling from 36 to 42 weeks. Comparison of our reference range with that of Moore and Cayle (1990) shows obvious differences at the lower limit.

Conclusion: We have established a normal longitudinal reference range for the amniotic fluid index. Our ranges, derived from longitudinal data, would seem a more appropriate reference than the established reference ranges derived from cross-sectional data. In particular, the differences at the lower limits may have considerable clinical implication in the use of amniotic fluid index in the prediction of fetal compromise.