The amniotic fluid index in late pregnancy.

Objective: Our purpose was to examine changes in the amniotic fluid index (AFI) in accurately dated term pregnancies both in relation to gestational age and in relation to the onset of spontaneous labor.
Study Design: This was a prospective observational study in 137 women with uneventful term pregnancies, in whom 220 AFI measurements were performed. More than one AFI value was available from 51 individuals.
Results: The AFI did not change significantly between 37 and 42 weeks' gestation, but a significant reduction was seen during the last 11 days before the spontaneous onset of labor (R = -0.37, n = 83, p < 0.001). The AFI (corrected for gestational age) within individuals remained stable over periods of up to 2 weeks. Meconium staining of the amniotic fluid was related to gestational age, but not to the AFI or fetal distress at birth. No significant correlation was found between fetal distress and the AFI, or between fetal distress and the reduction in AFI during the last two measurements before labor.
Conclusions: The reduction of the AFI in pregnancies progressing beyond term is related to the labor process itself rather than to the exact gestational age.