Objective: To investigate whether a relationship between birth weight (BW) and amniotic fluid index (AFI) existed for term patients with intact membranes and, if so, whether it could predict macrosomia and intrauterine growth retardation.
Study design: Two hundred thirty-one eligible women at term (37-42 weeks' gestation) had an AFI between 5.0 and 24.0 cm at the time of admission. Demographic information was collected, including BW. Statistical comparisons were made, with significance set at P < .05.
Results: The mean gestational age was 39.5 weeks, and the mean AFI was 11.3. There were 28 infants with BWs > 4,000 g (macrosomia). The cesarean section rate was 16.9%. Macrosomia was associated with a higher cesarean section rate (39.2% vs. 13.8%, P < .002). The mean AFI was higher for those with macrosomia (13.5 vs. 11.0, P < .001). Patients with an AFI > 15.0 had over twice the incidence of macrosomia (RR 2.72, 1.1-6.6, 32.1% vs. 14.8%, P < .027). The risk was even greater for them with an AFI > 18.0 (RR 6.4, 2.2-18.7, P < .002). A linear relationship was observed between AFI and BW (r2 .061, P < .0001). BW increased with increasing AFI. No relationship between AFI of 5-8 and BW < 2,500 g was found.
Conclusion: Increasing AFI correlates linearly with increasing BW, and macrosomia is associated with an increased incidence of cesarean delivery. AFI > 15 carries over double the risk of birth of a macrosomic infant, while AFI > 18 has a risk of over six times. No association between low normal AFI and a growth-restricted infant was found.