Ultrasound Useful for Shoulder Dystocia Detection.

VAIL, COLO. -- Fetuses at high risk for shoulder dystocia can be identified by late third-trimester ultrasound evaluation looking for body-to-head disproportion, Dr. John C. Hobbins said at a conference on obstetrics and gynecology sponsored. by the University of Colorado.

He cited a method developed by Dr. Bruce F. Cohen and coworkers at the University of California, Irvine. They found that when abdominal diameter exceeded biparietal diameter by 2.6 cm or more in a group of borderline macrosomic infants, the rate of shoulder dystocia was 30%; when the difference was less than 2.6 cm, the rate was zero.

The original study involved a small patient sample: just 31 infants with an estimated fetal weight of 3,800-4,200 g, all belonging to diabetic mothers who underwent ultrasound examination within 2 weeks of vaginal delivery.

"I've talked with the investigators, and they assure me their data are completely holding up with a larger number of patients, both diabetic and nondiabetic. And I have found this method anecdotally to be very useful. I think this is something that's going to be very helpful to you," said Dr. Hobbins, professor of ob.gyn. and chief of obstetrics at the university in Denver.

The average abdominal diameter can be arrived at either by dividing abdominal circumference by pi or by measuring the abdomen along the X and Y axes at the level of the umbilical part of the portal vein.

One potential limitation of the method is that biparietal diameter is affected by fetal position, the ob.gyn. noted.

Dr. Hobbins asserted that there may well be a place for prophylactic cesarean section in the setting of an estimated fetal weight in excess of 4,500 g coupled with body-to-head disproportion of 2.6 cm or greater, since these findings identify a subgroup at markedly ...