SAN DIEGO – Biometric findings on third-trimester ultrasound predicted increased risk for shoulder dystocia at term in a chart review of 132 pregnancies in women without diabetes.
The risk of shoulder dystocia doubled with every unit deviation from the mean for the ratio of femur length to head circumference and the ratio of head circumference to abdominal circumference, Dr. Stephanie M. Melka and her associates reported in a poster presentation at the annual meeting of the American College of Obstetricians and Gynecologists.
Other factors that independently predicted a statistically significant 4%-17% increased risk for shoulder dystocia at term included estimated fetal weight, femur length, abdominal circumference, and the ratio of femur length to biparietal diameter, reported Dr. Melka of Mount Sinai School of Medicine, New York.
The only standard measurements and ratios on prenatal ultrasound that were not significantly associated with increased risk for shoulder dystocia were head circumference and the ratio of femur length to abdominal circumference.
Several previous studies have shown that prenatal sonographic findings can predict shoulder dystocia at term in diabetic mothers, but this appears to be the first study of elective third-trimester sonography in low-risk nondiabetic women, the investigators noted.
The study identified 44 cases of shoulder dystocia during vaginal delivery at Mount Sinai in 2008-2009 in which the mothers underwent ultrasound examination early in the third trimester prior to 37 weeks. Each case was matched with two women who had a third-trimester ultrasound on the same day, for a total of 88 women in the control group.
The ratio of femur length to head circumference averaged 21 in the control group and 21.6 in the shoulder dystocia group. The ratio of head circumference to abdominal circumference averaged 1.05 in the control group and 1 in the dystocia group.
Biometric measurements and estimated fetal weight ascertained from third-trimester prenatal ultrasound can be used to counsel mothers on the risk for shoulder dystocia, the investigators concluded.
Other, classic risk factors for shoulder dystocia include maternal diabetes, obesity, a history of shoulder dystocia, and excessive weight gain during pregnancy.
The study did not control for the effects of obesity or weight gain on the risk for shoulder dystocia. It also did not assess the influence of differences in gestational age at the time of the sonograms.
Shoulder dystocia occurs in approximately 1% of all deliveries. Half of cases have been thought to be unpredictable.
Dr. Melka reported having no relevant financial disclosures.