1047: Increased abdominal circumference to head circumference ratio in late pregnancy is predictive of shoulder dystocia

Abstract

Shoulder dystocia is an obstetric emergency, which occurs in approximately 2% of vaginal births. Although it is more likely to occur in large for gestational age infants, sonographic estimated fetal weight alone is a poor predictor. Fetal body configuration with an increased body to head ratio may be more predictive than overall fetal size alone. The primary outcome of this study was to determine if abdominal circumference to head circumference (AC/HC) ratio >90th percentile is predictive of shoulder dystocia or other birth complications. This is a secondary analysis of the Genesis Study, a prospective, double-blinded, multi-center trial of 2,336 uncomplicated nulliparous patients with a vertex presentation. Participants had ultrasound and clinical evaluation performed between 39 0/7 and 40 6/7 weeks’ gestation. An AC/HC ratio was calculated for each participant and those >90th centile (n=239) were compared with those ≤90th centile (n=2095). In addition to the primary outcome, rates of emergency delivery and a composite of markers of birth injury including Erb’s palsy, cephalohematoma, fetal laceration, facial nerve palsy, fracture of the clavicle, skull or humerus were examined. Data were available for 2,334 of 2,336 patients; of these 21% (490/1844) required cesarean delivery. An AC/HC ratio >90th centile did not influence mode of delivery and carried a similar risk of emergency cesarean or operative vaginal delivery (p=0.47). The primary outcome, shoulder dystocia, occurred in 1.5% (29/1844) of vaginal deliveries and patients with an AC/HC ratio >90th centile were more likely to have a shoulder dystocia than those with a ratio ≤90th centile (4.5%[8/179] vs. 1.3%[21/1665]; p=0.001). Patients with an AC/HC ratio >90th centile had a longer first stage of labor (6.3±3.3 vs. 5.9±4.1 hours; p=0.02) and were also more likely to have a post-partum hemorrhage (12%[22/179] vs. 6.9%[115/1665]; p=0.009). There was no difference in rates of Apgar score <7 at 5 min, or arterial cord pH <7.20 between the two groups. An AC/HC ratio >90th centile did not convey an increased risk of birth injury after a composite of adverse outcomes was examined between both groups (3.4%[6/179] vs. 3.2% [54/1665]; p=0.93). An AC/HC ratio can be calculated using standard biometric parameters late in the third trimester. These data suggest use of this ratio may be helpful in decision-making regarding mode of delivery in those at above normal risk of shoulder dystocia.