The risks from vaginal delivery after a prior Cesarean delivery are low, but are slightly higher than for a repeat Cesarean delivery. This finding is from the largest, most comprehensive study of its kind ever conducted, undertaken by the National Institute of Child Health and Human Development of the National Institutes of Health.

The study appears in the December 16 New England Journal of Medicine.

"These findings provide women who have had a Cesarean delivery — and their physicians — with reliable information to take into account when deciding whether to undergo labor or to have a repeat Cesarean delivery," said Duane Alexander, M.D., Director of the NICHD.

Among the complications the study found in women who attempted a vaginal birth after prior Cesarean delivery were rupture of the uterus, infection of the uterine lining, lack of oxygen to the infant brain, and infant death. The study authors noted, however, that the risks of these complications were very low.

Cesarean delivery consists of delivering a baby through an incision made in the abdominal wall and through the uterus, rather than through the vagina. Reasons for Cesarean delivery include failure of labor to proceed normally, fetal heart rate abnormalities, and complications involving the placenta. Because cesarean delivery is a major surgical procedure, it carries the risks posed by any other major surgery, such as infection or complications from the anesthetic. Having a Cesarean delivery may also complicate future births.

Uterine rupture is the most well known complication of attempted vaginal delivery after a prior Cesarean delivery. Uterine rupture occurs when the scar in the uterine muscle opens. The rupture may result in part or all of the baby and perhaps the placenta leaving the uterus, which may cause fetal heart rate abnormalities and perhaps fetal death. A more severe, or catastrophic, rupture may result in heavy bleeding, which can endanger the lives of both mother and baby. In some cases, the bleeding may be so severe that a hysterectomy must be performed.

However, repeat Cesarean delivery also may carry risks beyond those posed by delivering vaginally after a prior cesarean delivery, explained the NICHD author of the study, Catherine Spong, M.D., Chief of the Institute's Pregnancy and Perinatology Branch. The risk for infection and other surgical complications appear to be greater in women undergoing repeat cesarean delivery compared to those who are successful with a vaginal birth after Cesarean delivery.

Moreover, having a repeat cesarean delivery may complicate future pregnancies, sometimes causing the placenta to implant over the cervix, thereby interfering with the birth process. Prior Cesarean also increases the chances that the placenta will grow into the uterine wall, leading to difficulty with removal of the placenta after the birth. This may result in heavy bleeding during birth, perhaps leading to surgical removal of the uterus.

The decision of whether to attempt a vaginal delivery or to have a repeat Cesarean must be made carefully by women and their physicians. They must take into account, on the one hand, the risk of uterine rupture and its attendant complications, and balance these factors against the risk of surgical complications and the chances that repeat Cesarean delivery might complicate future pregnancies.

Citing figures compiled by the National Center for Health Statistics (NCHS), the study authors noted that the rate of Cesarean delivery had increased from 5 percent in 1970 to an all time high of 26 percent in 2002. Recent preliminary data released by the NCHS indicated an overall Cesarean delivery rate exceeding 27 percent for 2003. For the same period, the rate of vaginal birth after Cesarean delivery had fallen from 31 percent in 1998 to 10.6 percent in 2003. The U.S. Public Health Service, in its Healthy People 2010 Report, proposed a target rate of vaginal birth after Cesarean delivery of 37 percent.

The NICHD Maternal-Fetal Medicine Units Network researchers undertook the current study to more precisely estimate the risks from vaginal birth after Cesarean delivery as compared to having a repeat Cesarean delivery. Before the current study, the only information on this topic was from studies that reviewed discharge codes from hospital records, Dr. Spong said. Such analyses, undertaken after the fact, may fail to include important information about the birth. Moreover, the few studies that had been conducted generally didn't include a large enough number of women for a reliable calculation of the risks involved.

The current study enrolled women at the 19 academic medical centers comprising the NICHD Maternal-Fetal Medicine Units Network. All of the women in the study were pregnant, and each had a previous Cesarean delivery. In all, 17,898 of the women attempted a vaginal birth and 15,801 underwent an elective repeat Cesarean delivery. Women were classified as having an elective Cesarean if they did not have a medical indication (need) for it and if they did not have labor.

Of the women who attempted a vaginal birth after cesarean delivery, only 0.7 percent, or 124 women in all, experienced a rupture of the uterus. The study also found that using drugs to induce or speed up labor may also increase the chances for uterine rupture. Such drugs increase the force and duration of uterine contractions. Of the 1864 women given the drug oxytocin alone, without any other drugs to induce labor, 1.1 percent (20 women) had a uterine rupture. None of the 227 women receiving the drugs known as prostaglandins alone experienced uterine rupture. Dr. Spong explained, however, that it's possible that the study sample did not include a sufficient number of women to determine a small increase in uterine rupture from prostaglandins alone.

Among the infants born to the women who attempted vaginal birth after a Cesarean, .08 percent (12) were diagnosed with hypoxic ischemic encephalopathy, a condition that may result from lack of oxygen to the baby's brain. The lack of oxygen may be caused by heavy maternal bleeding, detachment of the placenta, or other complications. Of these 12, seven were associated with a uterine rupture, and two of the babies died. In contrast, none of the infants whose mothers had an elective cesarean delivery developed hypoxic ischemic encephalopathy.

Among the women who attempted vaginal birth, the overall risk for either brain injury to the baby or death to the baby at term from uterine rupture was roughly 1 in 2000 trials of labor, said Mark B. Landon, M.D., of Ohio State University and the lead investigator for the NICHD Maternal-Fetal Medicine Units Network Cesarean Registry.

Women who attempted vaginal birth after cesarean were also more likely to develop infection of the uterine lining (2.9 percent) as compared to women who had an elective repeat Cesarean delivery (1.8 percent). The study authors found no significant difference between the percentage of women who required a hysterectomy: 0.2 percent in the labor group and 0.3 percent in the C-section group. Similarly, there was no significant difference in the maternal death rate between the two groups of women (.02 percent versus .04 percent.)

Dr. Spong said that the only way to arrive at a more accurate estimate of the risks involved would be to assign women at random to either a vaginal delivery or to have a repeat C-section. It's possible, she added, that the women who chose labor may differ in some unknown way from the women who had repeat Cesarean delivery, and that this difference might have influenced the study's results.

Still, Dr. Spong said, because of the large number of women who took part in study, and the careful, systematic way the researchers collected the data, the current study offers the most reliable estimate to date of the risks conveyed by attempting vaginal delivery after a prior Cesarean delivery.

The NICHD is part of the National Institutes of Health (NIH), the biomedical research arm of the federal government. NIH is an agency of the U.S. Department of Health and Human Services. The NICHD sponsors research on development, before and after birth; maternal, child, and family health; reproductive biology and population issues; and medical rehabilitation. NICHD publications, as well as information about the Institute, are available from the NICHD Web site, http://www.nichd.nih.gov, or from the NICHD Information Resource Center, 1-800-370-2943; e-mail NICHDInformationResourceCenter@mail.nih.gov.