A Vaginal Birth After Cesarean (VBAC) is the successful vaginal delivery of a baby following a previous cesarean section. The attempt to achieve this is called a Trial of Labor After Cesarean (TOLAC). Pursuing a TOLAC involves weighing the possibility of a VBAC against potential safety concerns. This choice requires assessing general statistics and the individual’s medical history, which modifies the likelihood of a successful outcome.
General Success Rates
For appropriate candidates, the probability of a successful VBAC is favorable. Studies consistently show that between 60% and 80% of women who attempt a TOLAC will achieve a VBAC. The overall success rate in the United States often centers around 70% for individuals with one prior low-transverse cesarean delivery.
TOLAC is the attempt to labor; VBAC is the successful result. Not every person who begins a TOLAC will achieve a VBAC, and the attempt carries specific risks that must be weighed against the benefits of avoiding major abdominal surgery. Individual factors are required for a more accurate prediction than the average success rate.
The baseline success rate of 70% indicates that most candidates will avoid a second surgery. A successful VBAC offers several advantages over an elective repeat cesarean delivery, including a faster recovery, a shorter hospital stay, and a lower risk of infection or significant blood loss. VBAC also helps avoid the cumulative risks associated with multiple cesarean surgeries, such as placenta accreta in future pregnancies.
Personalizing the Probability
The general success rate is heavily modified by clinical and historical data. The strongest positive factor predicting VBAC success is a history of any previous vaginal delivery. People who have delivered vaginally have a significantly higher success rate, often exceeding 90%, because their bodies have already demonstrated the ability to complete labor.
The reason for the previous cesarean delivery also plays a large role. A prior cesarean performed for a non-recurrent reason (e.g., fetal breech presentation or fetal distress) is associated with a much higher VBAC rate. Conversely, a primary cesarean due to recurrent issues (e.g., labor dystocia or “failure to progress”) slightly lowers the probability.
Maternal characteristics and current pregnancy details further refine the probability. Individuals with a lower Body Mass Index (BMI), a younger maternal age, and spontaneous onset of labor tend to have better outcomes. The use of labor induction or augmentation, or a gestational age beyond 40 weeks, decreases the success rate. These factors are integrated into predictive models to provide an estimated chance of success.
The type of incision used in the previous cesarean determines eligibility for a TOLAC. A low-transverse incision, which runs horizontally across the lower uterine segment, is required. A prior classical incision (a vertical cut in the upper uterus) is a near-absolute contraindication due to the high risk of uterine rupture. The integrity of the uterine scar is foundational to safety.
Uterine Rupture: The Primary Safety Metric
The main safety concern associated with a TOLAC is uterine rupture, a rare but serious complication. This occurs when the prior cesarean scar tears during labor, which can be life-threatening for both the birthing person and the fetus. For individuals with one prior low-transverse incision, the absolute risk of uterine rupture during a TOLAC is low, estimated at less than 1% (typically 0.5% to 0.9%).
A rupture can cause severe complications, including major hemorrhage, emergency hysterectomy, and serious injury or death to the baby due to decreased oxygen flow. Because of this potential for harm, a TOLAC must be attempted within a facility capable of providing immediate emergency care and continuous fetal monitoring. An abnormal fetal heart tracing is often the first sign of the complication.
Certain factors increase the risk of uterine rupture above the baseline rate. Labor induction, especially with prostaglandin agents, is associated with a higher risk compared to spontaneous labor, leading to restrictions on their use in TOLAC. An interval of less than 18 months between the prior cesarean and the current delivery is also linked to a higher rupture risk. Despite this risk, a successful VBAC is associated with fewer overall complications than an elective repeat cesarean delivery. The safety profile remains favorable for appropriate candidates managed in a setting with immediate surgical readiness.
Outcomes of a Failed Trial of Labor
A final consideration is the consequence of an unsuccessful trial ending in an unplanned cesarean delivery. A failed TOLAC means the birthing person labored but required an emergency repeat cesarean section. This outcome carries a higher risk of maternal complications compared to both a successful VBAC and a planned elective repeat cesarean delivery (ERCS). Increased risks include higher rates of postpartum hemorrhage, infection, blood transfusion, emergency hysterectomy, and intensive care unit admission. This elevation is attributed to the stress of labor, which increases the likelihood of complications during the subsequent surgery.
For the newborn, a failed TOLAC is associated with a greater chance of a low Apgar score and increased likelihood of admission to the neonatal intensive care unit. Maximizing the chance of a VBAC minimizes the exposure to the risks of a failed trial, making the probability of success an important measure in the full assessment.