(231) Third Trimester Ultrasound Fetal Abdominal Circumference Measurement and Risk of Neonatal Shoulder Dystocia at Birth

Fetal macrosomia is a well-established risk factor for neonatal shoulder dystocia (ShD). There is data that EFW > 4500g and abdominal circumference (AC)/head circumference (HC) ratio >90% is associated with shoulder dystocia, but risk of shoulder dystocia by AC parameter alone is scarce. We evaluated whether the risk of shoulder dystocia is associated with third trimester ultrasound measurement of AC >99%.

Study Design:

We investigated 208 mothers who delivered infants with ShD and 720 randomly selected controls who delivered vaginally from a cohort of women who received antenatal care at an urban, tertiary medical center from 2012 – 2023 for association of AC >99% and shoulder dystocia at delivery. Our secondary aim was to evaluate if other common risk factors of ShD are generalizable to our diverse, urban patient population. We used adjusted logistic regression to estimate the risk of ShD in those with AC >99%.

Results:

Pregnant women who had infants with ShD were more likely to be younger (26.8 vs 28.4; p< 0.001), nulliparous (1.1 vs 1.5; p=0.001), demonstrate greater weight gain in pregnancy (29.5 vs 24.7; p =0.02), report a history of prior shoulder dystocia (6.2% vs 2.2%; p=0.001), have diabetes (23.9% vs 14.6%; p=0.003) and undergo operative vaginal delivery (3.8% vs 1.7%; p=0.04). Patients with third trimester fetal AC >99% did not have a difference in their risk of shoulder dystocia compared to those with third trimester fetal AC < 99% (32.6% vs 28.6%; OR 1.2; p=0.29). Patients with EFW > 4500g on third trimester ultrasound were more likely to have shoulder dystocia compared to those with EFW < 4500g (2.6% vs 0.7%; OR 3.9; p=0.02).

Conclusion:

Fetal ultrasound measurements of AC > 99% in the third trimester are not associated with increased risk of shoulder dystocia. Other identified risk factors of shoulder dystocia such as maternal age, parity, weight gain in pregnancy, history of shoulder dystocia, diabetic status, EFW >4500g, and operative vaginal delivery are similar in this patient population.