In 2002, over a quarter of all births in the United States were cesareans. Most women who have cesareans have later babies the same way. Many women have been encouraged to try to have vaginal births after cesarean sections, but it's not clear that this is safe, because of the risk that the uterus will rupture under the stress of labor. A group of researchers from across the country looked at the risks of vaginal births after cesarean delivery, for the National Institute of Child Health and Human Development Maternal-Fetal Medicine Units Network.

What the researchers wanted to know: What are the risks to the mother and baby of a vaginal birth after cesarean delivery?

What they did: From 1999 to 2002 (or some part of that time), 19 academic hospitals collected data on 45,988 women who were having one baby and had previously had a cesarean delivery. For each woman, nurses went over her and her baby's medical records. Among other things, they were looking for uterine ruptures and babies who had seizures. Thirty-nine percent of the women attempted labor and 34 percent chose to have a cesarean section. (The rest didn't fit into the study for one reason or another—for example, having a medical reason for needing a cesarean the second time.)

What they found: None of the women who chose a cesarean birth had their uteruses rupture; 124, or 0.7 percent, of the women who tried labor had uterine rupture. Inducing labor or augmenting labor with oxytocin increased the risk of uterine rupture. Women who attempted labor were also more likely to experience endometritis or to need a transfusion. A few more women in the cesarean group died than women in the labor group, but so few women died (only 10 in total) that the differences weren't statistically significant. If their mothers tried labor, babies were more likely to be stillborn and to experience hypoxic-ischemic encephalopathy.

What the study means to you: Both women and their babies seem to be at higher risk from vaginal birth after cesarean delivery. The risks are not very large, but they're important for women to know about.

Caveats: The women weren't randomly assigned to either vaginal birth or cesarean delivery, and women who choose to try labor might be different from women who prefer a cesarean section.

Find out more: Information about C-section (with diagrams) from the National Library of Medicine

Read the article: Landon, M.B., et al. "Maternal and Perinatal Outcomes Associated with a Trial of Labor after Prior Cesarean Delivery." New England Journal of Medicine. Dec. 16, 2004, Vol. 351, No. 25, pp. 2581–2589.

Abstract online: http://content.nejm.org