Relationship between third-trimester sonographic estimate of fetal weight and mode of delivery

Objectives - Some have suggested, based on limited data, that knowledge of an estimated fetal weight from a sonogram in a low-risk population, particularly in the setting of a larger fetus, is associated with increased risk of cesarean delivery. We aimed to investigate, among women delivering neonates weighing greater than 3500 g, whether having had a sonographically estimated fetal weight in temporal proximity to delivery was associated with the risk of cesarean delivery. Methods - We conducted a retrospective cohort study of term nulliparous women delivering live-born, cephalic, singleton, nonanomalous fetuses with birth weights of greater than 3500 g. The study was powered to detect a 30% change in cesarean delivery frequency with the presence of a sonographic examination after 36 weeks' gestation. Results - Of the 2099 women meeting inclusion criteria, 419 (20%) had a sonographic examination after 36 weeks' gestation. Women were similar with respect to demographic and obstetric characteristics regardless of whether they underwent sonography. There were no differences in rates of cesarean delivery regardless of whether women had or did not undergo sonography after 36 weeks (33.2% versus 29.4%, respectively; P = .13). There also were no differences in rates of chorioamnionitis, postpartum hemorrhage, episiotomy, third- or fourth-degree perineal laceration, or neonatal adverse outcomes based on sonographic status. Findings were similar in a multivariable analysis, as well as when the study population was restricted to those with birth weights of greater than 4000 and 4500 g. Conclusions - In this population of neonates weighing greater than 3500 g, the presence of a sonographic examination was not associated with the frequency of cesarean delivery.