From 1970 to 2002 the rate of cesarean delivery in the United States rose from 5% to 26%, and it continues to climb. Since it was first recognized, efforts have been made to reduce the number of cesareans. These efforts included shifting away from the then prominent belief, “Once a cesarean, always a cesarean.” Instead, doctors were encouraged to recommend that women with a history of cesarean delivery attempt subsequent vaginal deliveries . Consequently, the proportion of vaginal births after cesareans (i.e. VBACs) rose from 3% in 1981 to 31% in 1998. However, this rate dropped back down to 13% by 2002, owing to an apparent increased risk of uterine rupture and concern about maternal and fetal mortality.

Until recently, however, no well-designed studies had provided conclusive evidence either for or against the observation that VBACs are less safe than repeat cesareans. Members of the NICHHD’s (National Institute of Child Health and Human Development) Maternal-Fetal Medicine Units Network, therefore, conducted a four-year, prospective study of over 30,000 women—all with a history of cesarean delivery—who gave birth either by trial of labor or by repeat cesarean. Their aim was to compare adverse outcomes for both mother and infant between the two methods.

The findings of this study, published in the December 16, 2004 issue of the New England Journal of Medicine , confirm earlier speculation: While absolute risk was low, VBAC was associated with a slightly greater risk of uterine rupture, uterine infection, and infant brain damage from lack of oxygen.

About the Study

The study was conducted from 1999 to 2002 at 19 academic medical centers in the US. All women with a history of cesarean delivery who were pregnant again with a single, normal fetus were considered for analysis. Of these 45,988 women, 17,898 (39%) underwent a trial of labor (i.e. a VBAC), while 15,801 (34%) had an elective repeat cesarean. The remaining women were excluded from the study because they either had a non-elective cesarean due to complications during pregnancy or labor, or had no documentation regarding their method of delivery.

Perinatal outcomes were compared between the two delivery groups. Adverse maternal outcomes included: uterine rupture or disruption, hysterectomy (removal of the uterus), blood clots, endometritis (infection of the uterine lining), blood transfusion, and death; adverse neonatal outcomes included: stillbirth, brain damage from low oxygen levels, and death.

Trained nurses reviewed medical records to determine perinatal outcomes, other details of the birth procedure, demographic data, and medical history. Information about newborns was collected for up to 120 days after the delivery.

The Findings

Results showed a slight, overall increased risk of both maternal and infant complications for women attempting vaginal births rather than repeat cesarean deliveries.

Vaginal Birth Success Rate

The overall success rate for vaginal delivery after cesarean (VBAC) was 73%, which means that 27% had to resort to cesarean section after a failed attempt at vaginal delivery. Among women who attempted a trial of labor, 124 (0.7%) experienced uterine rupture. When the drug oxytocin was administered to induce labor, risk of uterine rupture increased regardless of final delivery method.

Maternal Outcomes

Maternal endometritis and the need for blood transfusion were 1.6 and 1.7 times more common with VBAC than with repeat cesarean—a slight increase in risk. There was no significant difference in the risk of hysterectomy or maternal death between the two methods. Only ten out of 33,699 women died as a result of childbirth in this study—that's under 0.03%. Three of the women who died underwent a trial of labor while seven had a repeat cesarean; however, only two of these seven were directly attributed to the cesarean procedure itself. Only 88 women required hysterectomy. After adjusting for other relevant factors, the overall risk of an adverse maternal outcome was almost double (1.96 times greater) for those attempting a trial of labor compared to those who electively underwent repeat cesarean.

Infant Outcomes

Two infants died during delivery in this study, and 20 died soon after birth, but there was no statistic difference in fetal death rates between the two delivery methods. However, the incidence of brain damage from lack of oxygen was significantly higher among VBAC births as compared to cesarean births (12 vs. 0 occurrences). To put this increase risk into perspective, the researchers determined that it would take 588 repeat cesareans before one infant would be saved from an adverse outcome of vaginal birth.

In determining all the above results, the researchers controlled for many other factors that could influence their findings: maternal age, race or ethnicity, marital status, smoking status during pregnancy, type of insurance at time of delivery, underlying medical disease, number of previous cesarean deliveries, number of previous vaginal deliveries (if any at all), and birth weight of the current infant.

How Does This Affect You?

While the overall risk of severe complications from either method is low, this study does suggest that once a woman has a cesarean section, it is probably safer to stick with this method for subsequent births. However, the study does not suggest that VBAC’s should never be attempted. In fact, the chances of an unfavorable outcome was apparently quite small, and certainly worth discussing in some detail with your obstetrician ahead of time.

Another point to consider is that there may have been some bias in this study’s design. Those women who chose to attempt a vaginal birth after a previous cesarean may have had certain unidentified characteristics making them either more or less vulnerable to an adverse outcome.

Vaginal births are clearly safer for the vast majority of women who have never delivered before, or who have only delivered vaginally in the past. Efforts to reduce the number of cesareans in this population, therefore, are still appropriate and necessary. Given the results of this study, however, encouraging vaginal births after a cesarean is probably not the best to way to increase the number of vaginal deliveries.