Maternal Body Mass Index and Amniotic Fluid Index in Late Gestation

Objectives To determine the effect, if any, of an increasing maternal body mass index (BMI) on sonographically diagnosed oligohydramnios in late gestation and how it subsequently affects obstetric and neonatal outcomes. Methods This retrospective cohort study evaluated all women with singleton gestations who had a sonographic examination at 40 to 42 weeks' gestational age at North Shore University Hospital from 2010 through 2013. Underweight women (prepregnancy BMI < 18.5 kg/m2) were excluded because of higher rates of oligohydramnios and fetal growth restriction. Patients were classified into 5 groups by late-pregnancy BMI. The primary variable of interest was the diagnosis of oligohydramnios (amniotic fluid index < 5 cm). Secondary variables of interest included the mode of delivery and indication for primary cesarean delivery. A multivariable logistic regression analysis was performed. Results Oligohydramnios was identified in 189 of 1671 patients (11.3%). There was no significant difference in the amniotic fluid index between BMI groups. The rate of primary cesarean delivery increased with each successive BMI group (P < .001) such that women in the class III obesity group had an approximately 3-fold higher rate of primary cesarean delivery than women in the normal BMI group and a 2-fold higher rate than women in the overweight BMI group. In the final multivariable logistic regression model, a high BMI, nulliparity, and excessive gestational weight gain were associated with primary cesarean delivery. However, oligohydramnios did not contribute significantly to the model. Conclusions The maternal BMI is not associated with oligohydramnios in late gestation. An increasing maternal BMI significantly increases the risk of primary cesarean delivery.