Fetal Weight Does Not Help Predict VBAC Success.

Using weight estimates to determine who gets a trial of labor 'is not justified.'

ST. JOHN'S, NFLD. -- Fetal weight estimates based on ultrasound are not useful in determining which women with a history of C-section due to cephalopelvic disproportion should undergo a trial of labor, results of a large retrospective study suggest.

Contrary to widespread practice, estimating the size of the fetus and comparing it to that of the index neonate should not be used as a risk factor for counseling women about their chances of failing or succeeding at a vaginal birth after C-section (VBAC), Dr. Emmanuella Pare said at the annual meeting of the Society of Obstetricians and Gynecologists of Canada.

Dr. Pare of the University of Pennsylvania, Philadelphia, and her associates studied the medical records of 1,176 women who attempted VBAC at 1 of 17 medical centers between 1995 and 1999. All the women had a history of C-section due to cephalopelvic disproportion at term and had never undergone vaginal delivery.

Overall, 720 of the women had a vaginal delivery with a success rate of 61%, which falls within the 50%-8% range seen in the United States. On average, fetuses in the successful VBAC group weighed 3,484 g, compared with 3,609 g for fetuses in the failed VBAC group.

Patients in the successful VBAC group had neonates that on average weighed 142 g less than the initial neonate delivered by C-section. By contrast, women in the failed VBAC group had neonates who weighed 24 g more than the initial neonate. While the variation between the groups was statistically significant, it was not possible to determine cutoff values that would be clinically useful in everyday decisions about who should attempt a trial of labor.

"We could not identify a cutoff that yielded an acceptable trade-off between sensitivity and specificity," Dr. Pare said. And in the end, looking at fetal weight differences between the index neonate and the current fetus "is no better than flipping a coin," she said.