This is a great question as many women are under pressure from their care provider to go into labor as early as 37 weeks, but more commonly 40 weeks, or they will have a repeat cesarean.
Let’s first look at what ACOG says in their 2010 VBAC recommendations,
Studies evaluating the association of gestational age with VBAC outcomes have consistently demonstrated decreased VBAC rates in women who undertake TOLAC [trial of labor after cesarean] beyond 40 weeks of gestation (49, 75–77). Although one study has shown an increased risk of uterine rupture beyond 40 weeks of gestation (76), other studies, including the largest study that has evaluated this factor, have not found this association (77). Although chances of success may be lower in more advanced gestations, gestational age of greater than 40 weeks alone should not preclude TOLAC.
In reading the Guise 2010 Evidence Report, which was the basis of the 2010 NIH VBAC Conference, I was disappointed that most of the studies included in their analysis comprised of only a few hundred women. (More on why this is an issue later.) The largest study (Grobman, 2007 ) included 3,239 women. Thus, I personally take Guise’s conclusions with a grain of salt and look forward to future published research on the topic:
Relative to women with spontaneous labor, there was no increase in risk of rupture among those induced at term. However, the available evidence on women with induced labor after 40 weeks GA indicates an increased risk compared with spontaneous labor (risk difference 1.8 percent; 95 percent CI: 0.1 to 3.5 percent). The NNH in this group is 56 (for every 56 women greater than 40 weeks GA with IOL during a TOL [trial of labor], one additional rupture will occur compared with having spontaneous labor)…. These findings should be interpreted with caution, as one study found similarly increased rates of rupture with induction regardless of GA, while the other found increased risk in the group with greater than 40 weeks GA only.
In the class I teach, I review two studies that measured rupture rates after 40 weeks. Please note there are more studies out there and you can see a list at the end of ACOG’s VBAC recommendations as well as Guise 2010. I focus on two specific studies because they differentiated between uterine rupture and dehiscence, either controlled for induction/augmentation or reported the percentage of women induced/augmented, and one has over 5,000 women.
Coassolo (2005) is a nice sized study including over 11,000 women that concluded: uterine rupture was “not significantly increased… beyond estimated due date.”
6,907 labors before 40 weeks:
– 29% of those labors were induced/augmented
– 1% rupture rate
4,680 labors greater than or equal to 40 weeks:
– 33% of those labors were induced/augmented
– 1.1% rupture rate
1,643 labors greater than or equal to 41 weeks:
– No info on the % induced or augmented
– 1.5% rupture rate
Zelop (2001) is a smaller study, but it reported rupture rates for non-induced/augmented labors versus induced labors. Zelop found that uterine rupture “does not increase substantially after 40 weeks but is increased with induction of labor.”
1,214 labors 37-40 weeks: 0.5%
1,001 labors greater than 40 weeks: 1%
290 labors 37 – 40 weeks: 2.1%
270 labors greater than 40 weeks: 2.6%
While Zelop is great because it gives us the rate of non-induced/augmented rupture, Coassolo has the strength of numbers. The rate of rupture Zelop reports for its induced labors is likely not incredibly accurate as we have less than 300 labors in each group.
It’s important to remember when looking at any study on VBAC that you need at least 5,000 VBAC labors in order to accurately measure the event of uterine rupture that occurs about 0.5% of the time (non-induced/augmented VBAC after one prior low transverse CS per Landon 2004.) The problem with smaller studies is that since you don’t have enough women to accurate measure the incidence of rupture, you often get really small or really large stats as we see with Zelop’s reported 2.6% rupture rate for women past 40 weeks.
Coassolo, K. M., Stamilio, D. M., Pare, E., Peipert, J. F., Stevens, E., Nelson, D., et al. (2005). Safety and Efficacy of Vaginal Birth After Cesarean Attempts at or Beyond 40 Weeks Gestation. Obstetrics & Gynecology , 106, 700-6. Available from: http://journals.lww.com/greenjournal/Citation/2006/01000/Safety_and_Efficacy_of_Vaginal_Birth_After.43.aspx
Guise JM, Eden K, Emeis C, et al. Vaginal Birth After Cesarean: New Insights. Rockville (MD): Agency for Healthcare Research and Quality (US); 2010 Mar. (Evidence Reports/Technology Assessments, No. 191.) Available from: http://www.ncbi.nlm.nih.gov/books/NBK44571/
Zelop, C. M., Shipp, T. D., Cohen, A., Repke, J. T., & Liberman, E. (2001). Trial of labor after 40 weeks’ gestation in women with prior cesarean. Obstetrics & Gynecology , 97, 391-393. Available from: http://journals.lww.com/greenjournal/Fulltext/2001/03000/Trial_of_Labor_After_40_Weeks__Gestation_in_Women.13.aspx