Factors Influencing Successful Trial of Labor After Cesarean Delivery
Previous vaginal delivery—especially prior VBAC—is the single most powerful predictor of successful trial of labor after cesarean (TOLAC), with women having at least one prior vaginal birth achieving VBAC rates of 86.6% compared to only 60.9% in those without such history 1, 2.
Key Predictive Factors for VBAC Success
Strongest Positive Predictors
Prior vaginal delivery history:
- Women with any previous vaginal delivery have a 3.9-4.2 times higher likelihood of successful VBAC 2
- Those with prior VBAC specifically have 5.17-fold higher success rates, achieving 92-97% VBAC success 3
- This factor outweighs nearly all other considerations
Labor characteristics at presentation:
- Higher Bishop score (>4) is independently associated with successful TOLAC (OR 4.3) 4
- Spontaneous labor increases success 1.6-fold compared to induced labor 2
- Greater cervical dilation on admission correlates with higher success rates 1, 2
Maternal physical characteristics:
- Greater maternal height independently predicts success (OR 1.09 per cm) 5
- Smaller abdominal circumference favors success (OR 0.95) 5
Strongest Negative Predictors
Fetal size:
- Birth weight ≥4,000 g (8 lb 13 oz) significantly decreases VBAC likelihood 1, 2
- Women with infants <4,000 g have 2.0 times higher success rates 2
- Estimated fetal weight >3,500 g reduces VBAC chance by 70% 4
Labor induction:
- Oxytocin induction decreases success rates 1
- Labor induction reduces VBAC likelihood by 60% (OR 0.43) 4
- Spontaneous labor is strongly preferred for optimal outcomes
Maternal obesity:
- BMI ≥30 reduces overall success to 68.4% versus 79.6% in non-obese women 2
- Critical combination: Obese women requiring induction without prior vaginal delivery achieve only 44.2% VBAC success 2
Prior cesarean indication:
- Previous cesarean for dystocia/cephalopelvic disproportion/failure to progress significantly lowers success rates (OR 1.7 for non-dystocia indications) 2
Maternal age:
- Age >30 years independently associated with failed TOLAC (OR 0.57) 4
Race:
- Caucasian race shows higher success rates (OR 1.8) compared to other racial groups 2
Clinical Application Algorithm
Highest likelihood candidates (>85% success):
- Any prior vaginal delivery (especially prior VBAC)
- Spontaneous labor
- Bishop score >4
- Fetal weight <4,000 g
Moderate likelihood candidates (60-75% success):
- No prior vaginal delivery
- Spontaneous labor
- Normal BMI
- Prior cesarean indication other than dystocia
Lowest likelihood candidates (<45% success):
- BMI ≥30 + labor induction + no prior vaginal delivery
- This combination warrants serious consideration of elective repeat cesarean
Important Caveats
The AAFP guidelines emphasize that predictive scoring models have limited clinical value because they cannot reliably determine which individual women face higher cesarean risk 1. The overall TOLAC success rate is approximately 74% in the U.S., making it a reasonable option for most women with one or two prior cesareans 1.
Critical safety consideration: While evaluating success factors, remember that short-term maternal outcomes are as good or better with TOLAC/VBAC versus repeat cesarean, though perinatal mortality is slightly higher. However, repeat cesarean increases long-term risks of abnormal placentation, hysterectomy, and surgical complications 1.
Physicians should reevaluate the labor plan when patients present in labor, as elements affecting labor risks and vaginal delivery likelihood must be reassessed at that time rather than relying solely on prenatal predictions 1.