Maternal body mass index and oxytocin exposure in nulliparous women: is there an interaction associated with maternal and neonatal morbidities?

OBJECTIVE: Increasing body mass index (BMI) and administered doses of oxytocin are related to maternal and neonatal morbidities. We evaluated the effect of oxytocin dosage, BMI, and their interaction on select perinatal outcomes.

STUDY DESIGN: We retrospectively studied nulliparas with singletons ≥36 weeks' gestation who received oxytocin and reached the second stage of labor. Oxytocin dosage was defined by the oxytocin product ([maximum oxytocin dose] × [duration of oxytocin infusion]/100 (milliunits)). Outcomes included cesarean, postpartum hemorrhage, puerperal infection, and a composite of neonatal morbidities. Logistic regression produced odds ratios (OR) for BMI and oxytocin product. The final model included a BMI-oxytocin product interaction term.

RESULTS: One thousand two hundred and four women met enrollment criteria. Increasing BMI was associated with longer duration from first exam to delivery and higher rates of cesarean. Oxytocin dosage increased linearly with BMI (p < .001). There were increasing odds of all four outcomes as both BMI and oxytocin dosage increased (except cesarean with oxytocin). However, there was no significant interaction between BMI and oxytocin for any outcome.

CONCLUSIONS: BMI and oxytocin dosage are associated with select maternal and neonatal outcomes. However, the lack of interaction between BMI and oxytocin product suggests that the higher administered dose of oxytocin associated with increasing BMI does not synergistically potentiate maternal and neonatal morbidities.