Clinical Review

VBAC: When is it safe?

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When do risks outweigh benefits, in light of ACOG’s newly cautious advisory? What conditions call for extra concern?


 

References

KEY POINTS
  • Selection criteria useful for identifying candidates for VBAC include: a limit of 1 prior low-transverse cesarean, clinically adequate pelvis, no other uterine scars or previous rupture, and no contraindications.
  • Offer VBAC only if obstetric care and anesthesiology are available throughout active labor, in case emergency cesarean is necessary.
  • Single-layer uterine closure may increase the risk of rupture during subsequent labors.
  • Epidural anesthesia is safe for women undergoing a trial of labor.
A woman’s first cesarean may be more fateful than ever, because 1 low-transverse cesarean delivery is the new limit for a trial of labor in subsequent pregnancies, advises a 2004 practice bulletin from the American College of Obstetricians and Gynecologists (ACOG).1 The previous bulletin on vaginal birth after cesarean (VBAC) recommended a limit of 2.

The new bulletin reaffirms the previous recommendation that obstetric and anesthesia personnel be immediately available throughout active labor, in case emergency cesarean is necessary.

VBAC is still within the standard of care, but rates were declining even before the new bulletin was released: from a high of 28.3% in 1996 to 12.6% in 2002.2

Benefits of VBAC may outweigh the risks in most women with 1 previous low-transverse cesarean,3 but even with optimal facilities and personnel, numerous factors warrant special caution, according to recent studies I’ll review in this article.

Recent studies of risks and benefits

No randomized trials. ACOG notes,1 “Despite thousands of citations in the world’s literature, there are currently no randomized trials comparing maternal or neonatal outcomes for both repeat cesarean delivery and VBAC.”

Success rates are similar for gravidas with previous cesarean for a nonrecurring indication and those with no previous cesarean.4-6

Uterine rupture is more likely during a trial of labor, but the rate is usually below 1%.7-9

Other limiting factors may include labor augmentation and induction, maternal obesity, gestational age beyond 40 weeks, birth weight over 4,000 g, and an interdelivery interval of less than 19 months.10-17

When a trial of labor fails, women face a heightened risk of uterine rupture, hysterectomy, transfusion, and endometritis.3,4,9

Perinatal death is more likely during VBAC than planned repeat cesarean, although the death rate is usually less than 1%.3,8,9,18

Indications and contraindications

The TABLE outlines potential candidates, ineligible gravidas, resources needed, and situations that warrant caution.

Don’t assume: Check the previous operative note

It is all too easy to presume that a previous cesarean section at term was performed through a transverse incision in the lower uterine segment.

While this may be true in the majority of cases, the actual operative note may reveal information relevant to the delivery decision: an extensive tear of the uterine incision, previously unrecognized uterine anomalies, or the need to perform a classical or T-shaped incision to facilitate delivery of the infant.

For these reasons, review the actual operative report whenever possible before a trial of labor.

2 prior low-transverse incisions

While this is not an absolute contraindication to VBAC, in today’s cautious climate ACOG recommends VBAC proceed only when there is also a history of successful vaginal delivery.1,19 Otherwise, women with 2 or more previous cesareans should undergo repeat abdominal birth.

Prior low-vertical incision

Although successful VBACs have been reported in women with a prior low-vertical uterine incision, many experts feel that these incisions often extend superiorly into the upper uterus and thus increase the likelihood of uterine rupture in subsequent labors.20,21

Greater risk with single-layer closure

Single-layer uterine closure appears to increase the likelihood of rupture during subsequent labors.22 As a result, many physicians have returned to 2-layer closure of the lower transverse uterine incision. It is unclear whether single-layer closure is a contraindication to subsequent labor, but it does warrant caution due to a 4-fold increase in the risk of rupture.22

Discourage closely spaced gestations

The shorter the interval between deliveries, the more likely is uterine rupture during a trial of labor.23,24 For those considering a subsequent VBAC, I recommend trying to space their next delivery at least 18 months after cesarean birth.

Labor induction increases risk

Spontaneous labor leads to successful VBAC more often than does labor induction or augmentation. In addition, a recent study found 5 times the risk of uterine rupture when oxytocin was used to induce labor, compared with elective repeat cesarean—although the rate of rupture was less than 1% in both groups.25

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