VBAC Calculator (Vaginal Birth After Cesarean)

A score of 8 to 10 is associated with approximately 93% VBAC success. A score of 6 is associated with 89% success, 5 with 77%, 4 with 67%, and 3 with 60%. Scores of 0 to 2 correspond to about 49% success. The highest contribution comes from a prior vaginal delivery before the first cesarean (4 points) and favorable cervical effacement (2 points).

A non-recurring indication is a reason for the prior cesarean that is unlikely to happen again in a subsequent pregnancy, such as fetal malpresentation (breech, transverse lie), placenta previa, or maternal illness. A recurring indication, most commonly cephalopelvic disproportion (CPD) or failure to progress (dystocia), suggests that the pelvis may be too small for vaginal delivery again. Non-recurring indications add 1 point to the Flamm-Geiger score.

According to ACOG (2010, reaffirmed 2020), approximately 60 to 80 percent of women who undergo a trial of labor after cesarean (TOLAC) achieve a successful vaginal delivery. Success rates vary by center, patient selection, and support for labor. Women with a prior vaginal delivery have the highest success rates (85 to 90%), while those with only prior cesareans have lower rates (50 to 65%).

VBAC is associated with lower maternal morbidity compared to a repeat elective cesarean in most cases. The main risk is uterine rupture, which occurs in approximately 0.5 to 1% of TOLAC attempts (compared to 0.02% with elective repeat cesarean). If rupture occurs, it requires emergency surgery. ACOG recommends TOLAC only at facilities with immediate access to emergency cesarean and anesthesia. The calculator does not assess rupture risk, only VBAC success likelihood.

The Flamm-Geiger score was derived from women with one prior cesarean. For women with two prior cesareans, TOLAC is considered riskier due to increased uterine rupture risk (estimated 1.5 to 2%). Many centers will offer TOLAC for two prior low-transverse uterine incisions after individual risk counseling. This calculator does not apply to the two-prior-CS scenario, and you should discuss this directly with a maternal-fetal medicine specialist.

A score of 0 to 2 corresponds to an estimated 49% VBAC success rate in the Flamm-Geiger model, meaning about half of women with these factors who attempt TOLAC will deliver vaginally. This is not a contraindication to TOLAC, but most clinicians use this as part of shared decision-making. Factors that commonly produce a low score include age 40 or older, no prior vaginal delivery, and a recurring CS indication such as dystocia.

Prior vaginal delivery is the strongest predictor of VBAC success in the Flamm-Geiger model, contributing 4 points if the delivery occurred before the first cesarean, or 2 points if it occurred after a prior cesarean. A woman who delivered vaginally before her first cesarean has already demonstrated that her pelvis can accommodate a vaginal birth, which greatly increases the odds of a successful TOLAC.

TOLAC refers to allowing labor to occur in a woman who has had at least one prior cesarean delivery, with the intention of achieving vaginal birth (VBAC). TOLAC includes the labor attempt and all monitoring during labor. VBAC is the outcome when TOLAC succeeds in vaginal delivery. Not all TOLAC attempts result in VBAC; some end in repeat cesarean due to labor failure or other complications.

BMI is not part of the Flamm-Geiger score used in this calculator, but research shows that higher BMI is associated with lower VBAC success rates. A BMI above 30 is associated with success rates 10 to 20 percentage points lower than normal BMI. The Grobman 2007 MFMU antenatal model includes BMI as a continuous predictor. If your BMI is above 40, discuss this factor separately with your care team as it may significantly influence your individual TOLAC risk.

Cervical effacement and dilation at the time of hospital admission are assessed during a cervical exam when labor begins or when the patient presents to the hospital. These factors are not known before the onset of labor (unless a membrane sweep or pre-labor cervical check is done). Use Antenatal mode in this calculator to get a pre-labor estimate without cervical data; switch to Admission mode once you have cervical exam findings.

Yes. Oxytocin induction for TOLAC is associated with a slightly higher uterine rupture risk compared to spontaneous labor. It is also associated with lower VBAC success rates because the cervix may not be ready and dystocia can result. The Flamm-Geiger score does not include induction as a factor, but ACOG guidance notes that prostaglandins should generally be avoided for cervical ripening in TOLAC due to elevated rupture risk.

Candidates for TOLAC should have had one or two prior low-transverse uterine incisions. A prior classical (vertical) uterine incision, a low-vertical incision, or a T-incision carries a much higher uterine rupture risk (4 to 9%) and is generally a contraindication to TOLAC. This calculator assumes a prior low-transverse incision; the score does not apply if you have had a classical incision.