Studies of labor induction in women planning VBAC
consistently find decreased likelihood of VBAC and almost all show
increased likelihood of scar rupture. This makes sense. If the
woman's uterus isn't ready to labor, forcing it will result in
longer, harder contractions than she would have had on her own,
contractions that are both less likely to achieve the goal of
progressive labor and that put more stress over a longer time on
the uterine scar.
I have two studies looking at VBAC after 40 weeks gestation.
They agree that the risk of scar rupture is not increased for women
laboring spontaneously. One study found that increasing pregnancy
duration decreased the likelihood of VBAC (Coassolo 2005) while the
other did not (Zelop 2001). This may have to do with the
participating obstetricians' beliefs about the effect of pregnancy
duration than anything intrinsic to the woman. It is
probably not coincidental that the earlier study, done in a time
period when obs were more relaxed about VBAC, was the one finding
that longer duration did not make a difference.
Coassolo KM, Stamilio DM, Pare E, et al. Safety and efficacy of
vaginal birth after cesarean attempts at or beyond 40 weeks of
gestation. Obstet Gynecol 2005;106(4):700-6.
Zelop CM, Shipp TD, Cohen A, et al. Trial of labor after 40
weeks' gestation in women with prior cesarean. Obstet Gynecol