A Vaginal Birth After Cesarean (VBAC) is the successful outcome of a Trial of Labor After Cesarean (TOLAC), where a woman attempts to deliver a baby vaginally following a previous Cesarean delivery. The decision to attempt a VBAC is complex, with a successful outcome offering benefits like a quicker recovery and avoidance of major abdominal surgery. When labor does not begin spontaneously, the question of whether to induce labor for a TOLAC arises, adding a layer of medical complexity to the process. This decision is not taken lightly and requires a specialized evaluation of individual patient risks and likelihood of success.
Understanding the Specific Risks of Inducing Labor After Cesarean
Inducing labor in a patient with a prior Cesarean section introduces a unique set of risks compared to induction in a patient with an unscarred uterus. The primary concern is the potential for uterine rupture, which is the separation of the previous C-section scar under the stress of strong contractions. While the absolute risk of uterine rupture during a spontaneous TOLAC is low, estimated to be around 0.52% (or 52 ruptures per 10,000 attempts), induction substantially modifies this baseline risk.
The likelihood of a uterine rupture increases with the intensity of the induced contractions, which can be difficult to control. For women undergoing a TOLAC, the risk of rupture with an induced labor using methods other than prostaglandins rises to approximately 0.77%. Uterine rupture, although rare, can be catastrophic, leading to severe outcomes for both the mother and baby and requiring immediate emergency surgery.
Approved and Avoided Induction Methods for TOLAC
The methods used to start labor are carefully selected for TOLAC candidates to balance the need for induction against the risk of scar rupture. Mechanical methods are generally considered a safer option for cervical ripening, as they work by physically dilating the cervix rather than chemically inducing strong contractions. The Foley catheter or a double-balloon catheter, which places direct pressure on the cervix to encourage dilation, is a frequently used mechanical method.
Once the cervix is ready, or if cervical ripening is not needed, low-dose Oxytocin (Pitocin) may be administered via an intravenous drip to stimulate contractions. This synthetic form of the hormone oxytocin is used cautiously and is closely monitored to prevent uterine hyperstimulation, which is excessively strong or frequent contractions. Artificial rupture of membranes (amniotomy) is another method that can be used to progress labor, often in conjunction with Oxytocin.
In contrast, pharmacological agents known as prostaglandins, such as misoprostol (Cytotec) and dinoprostone (Cervidil), are generally avoided or strictly contraindicated for induction in TOLAC patients. Prostaglandins are highly effective at causing cervical change and stimulating contractions, but they carry a significantly higher risk of uterine rupture. Studies have shown the rupture rate can be as high as 2.45% when prostaglandins are used for induction, which is a considerable increase over the spontaneous rate.
Patient Eligibility Criteria for Induced VBAC Success
The decision to proceed with an induced TOLAC rests on a thorough assessment of the patient’s likelihood of achieving a successful VBAC, which must be high enough to justify the increased risks. A key predictive tool is the Bishop Score, which evaluates the readiness of the cervix based on five factors:
- Dilation
- Effacement
- Consistency
- Position
- Fetal station
A higher score, typically 6 or more, indicates a more favorable cervix and a greater chance of successful induction and VBAC.
Other elements of a patient’s medical history are important in determining eligibility. The type of uterine incision from the previous Cesarean is paramount; a low transverse incision permits a TOLAC, while a classical (vertical) or T-incision is a strict contraindication due to a much higher rupture risk. The reason for the previous Cesarean also plays a role, as a non-recurrent issue like a breech presentation suggests a higher chance of success than a previous failure to progress in labor.
A history of a prior successful vaginal delivery is one of the strongest predictors of a successful VBAC, significantly increasing the odds. The time elapsed since the last delivery, known as the inter-delivery interval, is also considered; an interval of less than 18 months potentially increases the risk of uterine rupture. Additional maternal factors that can decrease the likelihood of success include advanced maternal age, a high Body Mass Index (BMI), and a gestational age past 40 weeks.