Association of amniotic fluid index with estimated fetal weight

The relationship between amniotic fluid volume and gestational age has been described previously. The association of body weight and urine output has been observed in human neonates. Our goal was to assess the correlation of the amniotic fluid index (AFI) with estimated fetal weight (EFW) in the third trimester.

We conducted a retrospective observational study on 426 pregnant women with singleton gestations who were referred to our unit for sonographic evaluation in the third trimester. The AFI, EFW, and EFW percentile corrected for gestational age were evaluated. The sonographic examinations were stratified into 3 gestational age categories: 28 through 33.9 weeks, 34 through 37.9 weeks, and 38 weeks and later. Maternal and fetal outcome variables were collected from medical records. Linear regression, Mann-Whitney U, and Kruskal-Wallis tests were used for statistical analysis.

There was no significant relationship between the AFI and EFW in the entire group of patients (R = 0.08; P = .096). There was a significant relationship between the AFI and EFW after 38 weeks' gestation (R = 0.30; P = .003). In addition, in female fetuses the EFW percentile correlated with higher AFI values at all gestational ages (R = 0.31; P < .001); this, however, was not observed in male fetuses.

There is no relationship between the AFI and EFW during the third trimester, although a positive relationship between the AFI and EFW was noted late in gestation. In pregnancies with female fetuses, the AFI was positively associated with EFW percentile before 38 weeks' gestation.