SAN FRANCISCO – Few women in a general U.S. obstetric practice are given a trial of labor after a previous cesarean birth, but the majority do have a vaginal delivery, a study has shown.

A team led by Dr. Kirsten Salmeen, a fellow in maternal-fetal medicine at the University of California, San Francisco, retrospectively studied a cohort of more than 1.1 million nonanomalous singleton term pregnancies in women with a history of cesarean delivery. The results, reported in a poster session at the Pregnancy Meeting, the annual meeting of the Society for Maternal-Fetal Medicine, showed that one in eight women were given a trial of labor. But two-thirds of this group succeeded in having a vaginal delivery.

"Overall, VBAC [vaginal birth after cesarean] rates are high" in routine clinical practice – "maybe not quite as high as in studies of VBAC, but still, very good success rates overall. However, the number of women attempting a trial of labor in this country is very, very low compared to the women who are likely to succeed," Dr. Salmeen said in an interview.

Resistance to a trial of labor after cesarean is complicated and multifactorial.

"Sometimes hospital policy comes into it. Sometimes provider comfort," she said. "Also, there is a lot out there among women in the community about the safety and the benefits and the pros and the cons. I think that a lot of women who are pregnant ... are sort of under the impression that it’s much safer for them to have a C-section. Starting to chisel away at that myth and really trying to educate women might go far."

Women in the study had especially good odds of having a vaginal delivery if they had had at least one previous vaginal delivery.

For hospitals that have policies against a trial of labor, or that are prohibitive in terms of a trial of labor, one category that can be viewed very differently is that of women with a history of previous vaginal birth, Dr. Salmeen said. She suggested that hospitals adjust their policies accordingly. If a woman has a 6.2 increase in the odds of having a successful vaginal birth, and nearly 90% of such women go on to have a successful VBAC – as was the case for those who had had at least three previous vaginal births – "maybe we can adjust those policies a little bit for those women," Dr. Salmeen said.

On the other hand, women had reduced odds of having a vaginal delivery if they had certain common medical conditions, but the reduction was relatively small, she noted.

In the case of gestational diabetes, for example, "a lot of people assume that those women are much less likely to have a successful VBAC, and it’s a very big part of our population," she said. Given their adjusted odds of success, "they were about 20% less likely to succeed than women who didn’t have gestational diabetes. But they weren’t, say, 80% or 90% less likely to succeed. So it was relatively small."

In the larger context, new models incorporating factors such as these may go a long way in predicting a woman’s odds of successful VBAC. "This data set has the potential to sort of test some of those models and see if they really stand up in a national population," Dr. Salmeen maintained.

She and her coinvestigators analyzed data from the U.S. Certificate of Live Birth data set for women giving birth between 2005 and 2009. Results reported at the meeting were based on 1,162,197 pregnancies among women having at least one previous cesarean delivery.

Overall, 13% had a trial of labor, reported Dr. Salmeen.

In a multivariate analysis, women were significantly more likely to be given a trial if they were black (odds ratio, 1.17); had less than a high school education (1.20); and especially if they had had one, two, or three or more previous vaginal deliveries (2.69, 4.22, and 6.09, respectively).

They were significantly less likely to be given a trial of labor if they were younger than age 20 (0.91) or aged 35 or older (0.95), were Hispanic (0.95), had pregestational diabetes (0.67) or gestational diabetes (0.81), or had chronic hypertension (0.71) or gestational hypertension (0.82).

Overall, 67% of the group given a trial of labor had a vaginal birth. This compares with roughly 70% in studies, Dr. Salmeen noted.

In a multivariate analysis, women were significantly more likely to succeed in being delivered vaginally if they had less than a high school education (odds ratio, 1.28) and especially if they had had one, two, or three or more previous vaginal deliveries (2.97, 4.50, and 6.23, respectively).

They were significantly less likely to succeed in being delivered vaginally if they were black, Hispanic, or Asian (odds ratios, 0.80, 0.92, and 0.86); were younger than age 20 (0.72); had pregestational diabetes (0.50) or gestational diabetes (0.81); or had chronic hypertension (0.56) or gestational hypertension (0.70).

Trial of labor after C-section uncommon, but often successful