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.