The four-quadrant sum of amniotic fluid pockets (amniotic fluid index) was studied prospectively in 791 normal pregnancies. Interobserver and intraobserver variation was 3.1% and 6.7%, respectively. Logarithmic transformations were used to establish the mean and 90% confidence intervals for the amniotic fluid index at each week of gestation. In term pregnancies, the boundaries of the amniotic fluid index were 115 mm (mean), 68 to 196 mm (5th to 95th percentiles). In postdates pregnancies greater than 42 weeks, the values were 108 mm (mean), 67 to 174 mm (5th to 95th percentiles), p less than 0.0001. However, the values for each week were statistically distinct, indicating the need to reference amniotic fluid index measurements to week-specific normative tables for accurate interpretation. This study provides normative data for the amniotic fluid index throughout pregnancy.