Amniotic fluid index. Gestational age-specific values for normal human pregnancy.

Amniotic fluid volume is an important parameter in the assessment of fetal well-being. The purpose of this study was to define the values of the amniotic fluid index for normal pregnancy by week of gestation in our population. The amniotic fluid index was measured prospectively in 892 patients with a normal singleton pregnancy between 15 and 40 weeks and an estimated fetal weight between the 10th and 90th percentile. The results were stratified by week of gestation. From a median of 10.3 cm (range, 8.7-13.7, 5th-95th percentile) at 15 weeks' gestation, the amniotic fluid index rose progressively to a maximum median of 14.0 cm (range, 4.0-18.6) at 30 weeks. The index then gradually declined to a median of 9.1 cm (range, 4.8-14.2) by 40 weeks' gestation. The difference between the median index for preterm patients (11.9 cm) and that for the term group (10.8 cm) was found to be statistically significant (P < .05). The difference between the median amniotic fluid index for the total group and the medians for the preterm and term patients was also significant (P < .05). Gestational age-specific values of amniotic fluid index should be used, and the 5th and 95th percentiles serve as the lower and upper limits, respectively, of normal.