Type of prior hysterotomy influences the VBAC decision
A classical uterine incision is an absolute contraindication to vaginal birth after cesarean (VBAC). A trial of labor is thought to be safe in women who have had a low transverse hysterotomy. The jury is still out on the safety of VBAC in a woman who has had a low vertical incision, however, because of uncertainty over whether the contractile portion of the uterus is involved.
Are multiple cesareans a contraindication to VBAC?
Experts disagree as to whether more than one previous cesarean delivery before a trial of labor increases the risk of uterine rupture. One retrospective study showed no difference in the rate of rupture between women who had a single previous cesarean and those who had more than one.7 A larger prospective study showed a modest increase in the risk of rupture (OR, 1.16) among women who had undergone more than one cesarean—but no decrease in the chance of success.8
Most large retrospective and prospective studies include patients who have had more than one previous cesarean delivery, but their numbers remain low; therefore, statistical significance cannot be determined.
Induction or augmentation of labor may lower odds of success
The likelihood of successful VBAC may be reduced when labor is augmented or induced. The picture is unclear because most studies that have focused on cervical ripening and induction of labor in VBAC are small.
Bujold compared pregnancy outcomes of three groups of women:
- those who underwent cervical ripening via Foley catheter
- those who had amniotomy and oxytocin administration
- those who entered labor spontaneously.
No difference in the rate of uterine rupture was found among the groups. However, the group that underwent cervical ripening had a significantly lower rate of success.9
A large case-control study found no increase in the rate of rupture when oxytocin or prostaglandins were administered, but the rate tripled when both were used together.10
A small, nested, case-control study found an increased risk of uterine rupture only when oxytocin was administered at a rate exceeding 20 mU/mL.11
More than 90% of hysterotomies are transverse
When the obstetric history is incomplete, the clinician may not know what type of hysterotomy was used in the previous cesarean delivery. Most experts believe that VBAC is acceptable when the previous cesarean involved a low transverse hysterotomy. The risk may be much higher with other types of incisions. Today, however, with modern techniques in place, we can assume that more than 90% of hysterotomies are of the low transverse type.
At least one study suggests that the risk of uterine rupture during vaginal birth after cesarean is acceptably low when the type of hysterotomy is unknown. That study explored the effect of augmentation of labor with oxytocin among women who had an unknown scar and found an increased risk of rupture, compared with women who were managed expectantly. However, the overall rate of uterine rupture did not differ from the rate expected when the hysterotomy is known to be of the low transverse type.12
VBAC for twins is rare
Because few women carrying twins attempt VBAC, we have little data to guide counseling on success and complication rates. A multicenter, retrospective, cohort study explored delivery outcomes of 25,005 women who had undergone at least one previous cesarean. Of these women, 24,307 had a singleton pregnancy, and 535 were carrying twins. Women who had a twin gestation were 40% less likely to attempt a trial of labor, but those who did had a chance of success and risk of uterine rupture similar to those of women with a singleton gestation. Women carrying twins who underwent a trial of labor had an elevated risk of requiring transfusion, compared with those carrying singletons, but this risk was similar to that of women delivering twins by elective repeat cesarean. In fact, women who delivered twins by repeat cesarean tended to have more maternal morbidity overall than those who had a trial of labor.13
A short interpregnancy interval precludes VBAC
Data indicate that a trial of labor after cesarean should be avoided in women who have a brief interpregnancy interval. Several retrospective studies had found an increased risk of uterine rupture, as well as a host of other adverse outcomes, among these women. Using 12 months as a reference point, women who had an interpregnancy interval shorter than 6 months had triple the risk of uterine rupture.14 Although the mechanism is unknown, rupture is presumably the result of incomplete healing of the hysterotomy.
Macrosomia may not increase the risk of rupture