Normal Values for Amniotic Fluid Index at 26-40 Weeks' Gestation

Abstract

Objective To establish the gestational age trends of amniotic fluid index (AFI) in uncomplicated pregnancy.

Design Prospective descriptive study. Setting Division of Maternal Fetal Medicine, Faculty of Medicine, Chulalongkorn University.

Subjects AFI was measured in 134 uncomplicated pregnancies seen between December 1996 and October 1997 and meeting the following criteria : no maternal medical or obstetric complications, normal growth of fetus by serial ultrasound. AFI was calculated by adding the deepest vertical pockets in each of four quadrants. Using the maternal umbilicus as reference, the uterus was divided into upper and lower halves. The linea nigra was then used as the midline to divide the uterus into right and left halves. The relation between gestational age and AFI was evaluated.

Results Of 134 uncomplicated pregnancies, 400 AFI measurements were performed between 26 and 40 weeks gestation. Percentile, mean, standard deviation of the AFI were determined according to gestational ages. The relationships of AFI to gestational age were observed to be linear. The regression model could be presented as AFI (mm.) = 226.483-3.0331 x GA (week) (r = 0.281).

Conclusion Gestational age trend in normative AFI measurements for our population has been established. Gestational age-specific values of AFI should be used to define abnormal amniotic fluid conditions.