Longitudinal study of the amniotic fluid index in post-dates pregnancy

Amniotic fluid (AF) was measured in 511 post-dates pregnancies (at least 41 weeks of gestational age) with the use of the AF index. Ultrasonographic evaluations were conducted on a semiweekly basis. Only patients with reliable gestational ages calculated from certain last menstrual period and confirmed by early sonographic estimates participated in the study. Oligohydramnios (AF index of 5.0 cm or less) was detected in 11.5% of the study population. Longitudinal data were available from 121 patients who demonstrated a mean 25% decrease in AF index per week beyond 41 weeks' gestation. The longitudinal change in AF index was statistically significant (P less than .0005). Amniotic fluid index measurements ranged from 1.7-24.6 cm, with a mean of 12.4 at 41 weeks' gestation. Compared with previous cross-sectional studies, this longitudinal study provides a more accurate estimate of changes in AF levels as a function of gestational age.