VBAC Success Probability
Simplified clinical estimate. For formal scoring use validated tools (MFMU calculator).
Prior CS Indication
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Vaginal birth after caesarean (VBAC) refers to a planned attempt at vaginal delivery in a woman who has had one or more previous caesarean sections. The clinical decision of whether to plan a trial of labour after caesarean (TOLAC) versus elective repeat caesarean section (ERCS) is one of the most consequential in obstetric care, balancing the benefits of vaginal birth against the risk of uterine rupture — a rare but potentially catastrophic complication. The Grobman nomogram, derived from a large prospective study of over 14,000 women, predicts the probability of successful VBAC at the time of admission for labour based on clinical and demographic factors collected in the antepartum period. The model includes: maternal age, BMI, ethnicity, presence or absence of a prior vaginal delivery (including prior VBAC), the indication for the previous caesarean (recurring versus non-recurring), and cervical examination findings on admission. Overall, approximately 60-80% of women who attempt TOLAC achieve vaginal delivery, with success rates varying substantially based on these predictors. The risk of uterine rupture in TOLAC is approximately 0.5-1% for women with a single previous low transverse caesarean scar, which is lower than the cumulative maternal morbidity of multiple repeat caesarean sections. Shared decision-making based on individualised risk prediction, including access to an emergency caesarean within 30 minutes if needed, is the cornerstone of VBAC planning. The VBAC calculator is an evidence-based conversation tool that supports this dialogue.
Grobman VBAC Success Score = logistic regression model incorporating: age, BMI, ethnicity (African-American and Hispanic reduce probability), prior vaginal delivery (yes/no), prior VBAC (yes/no), non-recurrent caesarean indication (yes/no), recurrent indication (yes/no), cervical dilation on admission (cm); Output: % probability of successful VBAC (60-80% overall range)
- 1Confirm the indication and type of previous caesarean: a single previous low transverse uterine incision is the standard TOLAC requirement. Classical (vertical) uterine incision or previous uterine rupture is a contraindication to TOLAC.
- 2Identify the indication for previous caesarean: non-recurring indications (foetal malpresentation, placenta praevia, cord prolapse) do not increase recurrence risk; recurring indications (failure to progress at full dilation, failure to descend) may recur.
- 3Collect maternal characteristics: age at delivery, BMI, and ethnic group. Older age, higher BMI, and certain ethnicities (African-American, Hispanic in Grobman dataset) are associated with lower VBAC success rates.
- 4Determine whether there is a prior vaginal delivery (including prior VBAC): prior vaginal delivery dramatically increases VBAC success probability. Prior VBAC specifically is the strongest positive predictor.
- 5Assess cervical examination on admission: dilation ≥4 cm on admission is a positive predictor of VBAC success.
- 6Apply the Grobman model (via validated calculator) to generate an individualised percentage probability of successful VBAC. Discuss this probability alongside the approximately 0.5-1% risk of uterine rupture and the maternal and neonatal implications of both TOLAC and ERCS.
- 7Ensure institutional readiness: TOLAC requires immediate access to operative delivery, continuous electronic fetal monitoring, and anaesthetic and paediatric support. Document the discussion and decision in the clinical record.
Strong candidate for TOLAC; prior vaginal delivery and non-recurring indication are most favourable factors
A prior vaginal delivery (including prior VBAC) is the single strongest predictor of VBAC success. Non-recurring indication (breech) means the anatomical reason for the first caesarean is absent. This woman has excellent VBAC odds.
Borderline candidate; thorough shared decision-making essential; discuss uterine rupture risk vs ERCS risks
Recurring indication (failure to progress), no prior vaginal birth, higher BMI, older age, and ethnicity all reduce the probability. Both options (TOLAC and ERCS) carry meaningful risks — the choice hinges on patient values and preferences.
Most guidelines support TOLAC after two prior CS but success rate is lower; ERCS is often recommended
Two prior caesarean sections with the same recurring indication significantly reduce VBAC success probability and increase uterine rupture risk. Many obstetricians would recommend ERCS, though TOLAC remains the patient's right after informed consent.
Strong TOLAC candidate; in many centres this woman would not require additional counselling beyond routine
A prior VBAC demonstrates that the uterine scar can withstand labour. The rupture risk is substantially lower than for a first TOLAC attempt, and the likelihood of vaginal delivery is very high.
Antenatal VBAC counselling clinic: the Grobman calculator provides an individualised probability to anchor the shared decision-making conversation., representing an important application area for the Vbac Calculator in professional and analytical contexts where accurate vbac ulator calculations directly support informed decision-making, strategic planning, and performance optimization
Labour ward admission: reassessing VBAC probability at cervical examination on admission when dilation is known., representing an important application area for the Vbac Calculator in professional and analytical contexts where accurate vbac ulator calculations directly support informed decision-making, strategic planning, and performance optimization
Risk stratification: identifying very high-probability VBAC candidates (>80%) who can be managed with standard monitoring versus borderline candidates requiring enhanced surveillance., representing an important application area for the Vbac Calculator in professional and analytical contexts where accurate vbac ulator calculations directly support informed decision-making, strategic planning, and performance optimization
Audit and quality improvement: tracking TOLAC success rates against predicted probabilities across obstetric units to benchmark counselling quality., representing an important application area for the Vbac Calculator in professional and analytical contexts where accurate vbac ulator calculations directly support informed decision-making, strategic planning, and performance optimization
Academic researchers and university faculty use the Vbac Calculator for empirical studies, thesis research, and peer-reviewed publications requiring rigorous quantitative vbac ulator analysis across controlled experimental conditions and comparative studies
VBAC with Unknown Uterine Scar
{'title': 'VBAC with Unknown Uterine Scar', 'body': 'When operative notes from a previous caesarean are unavailable, the type of uterine incision is unknown. In practice, the vast majority of caesarean sections use a low transverse incision. If there are no clinical or documentation cues suggesting a vertical incision (e.g., very early preterm caesarean, transverse lie, or placenta praevia with anterior implantation), a low transverse incision can usually be assumed. Senior obstetrician review is essential.'}
Short Inter-Delivery Interval
{'title': 'Short Inter-Delivery Interval', 'body': "Women with an inter-delivery interval of less than 18 months should be counselled that their uterine rupture risk during TOLAC is approximately 3 times higher than for women with longer intervals. This is attributed to incomplete myometrial healing. ERCS is often recommended in these cases, though TOLAC remains a patient's right after fully informed discussion."}
Previous Uterine Surgery (Myomectomy)
In the Vbac Calculator, this scenario requires additional caution when interpreting vbac ulator results. The standard formula may not fully account for all factors present in this edge case, and supplementary analysis or expert consultation may be warranted. Professional best practice involves documenting assumptions, running sensitivity analyses, and cross-referencing results with alternative methods when vbac ulator calculations fall into non-standard territory.
Grand Multiparity and VBAC
In the Vbac Calculator, this scenario requires additional caution when interpreting vbac ulator results. The standard formula may not fully account for all factors present in this edge case, and supplementary analysis or expert consultation may be warranted. Professional best practice involves documenting assumptions, running sensitivity analyses, and cross-referencing results with alternative methods when vbac ulator calculations fall into non-standard territory.
| Factor | Effect on VBAC Probability |
|---|---|
| Prior vaginal delivery | Strong positive — largest single predictor |
| Prior VBAC specifically | Very strong positive |
| Non-recurring CS indication | Positive |
| Cervical dilation ≥4 cm on admission | Positive |
| BMI <30 | Positive |
| Age <35 | Positive |
| African-American or Hispanic ethnicity (Grobman) | Modest negative |
| BMI >40 | Negative |
| Recurring CS indication (e.g., failure to progress) | Negative |
| No prior vaginal delivery | Negative |
| Two prior caesarean sections | Negative (also increases rupture risk) |
What is the overall VBAC success rate?
Approximately 60-80% of women who attempt TOLAC achieve vaginal delivery. The rate varies substantially with individual predictors: women with a prior vaginal delivery and a non-recurring caesarean indication have success rates above 85%, while women with no prior vaginal birth and a recurring indication may have rates below 50%. This is particularly important in the context of vbac calculatorulator calculations, where accuracy directly impacts decision-making. Professionals across multiple industries rely on precise vbac calculatorulator computations to validate assumptions, optimize processes, and ensure compliance with applicable standards. Understanding the underlying methodology helps users interpret results correctly and identify when additional analysis may be warranted.
What is the risk of uterine rupture in TOLAC?
The risk of symptomatic uterine rupture in women with one prior low transverse caesarean attempting TOLAC is approximately 0.5-1% (1 in 100-200 women). The risk is higher with two prior caesarean sections (~1.5%), after single-layer uterine closure, with a short inter-delivery interval (<18 months), and with oxytocin augmentation. The risk is lower in women with a prior vaginal delivery.
What are the signs of uterine rupture?
Classic signs include sudden change in fetal heart rate pattern (prolonged deceleration or bradycardia — most common sign), abrupt cessation of uterine contractions, maternal abdominal pain, and haematuria. Less specific signs include maternal tachycardia and suprapubic pain. Uterine rupture can present subtly; continuous electronic fetal monitoring throughout TOLAC is mandatory to detect the fetal heart rate changes that precede maternal deterioration.
Is oxytocin safe in TOLAC?
Oxytocin can be used for augmentation in TOLAC but increases uterine rupture risk approximately 2-3 fold. The lowest effective dose should be used. Prostaglandins for cervical ripening (misoprostol, dinoprostone) are generally contraindicated or used with extreme caution in women with a uterine scar as they further increase rupture risk. Mechanical ripening (Foley balloon) is preferred if ripening is needed.
What are the contraindications to TOLAC?
Absolute contraindications include: classical (vertical) uterine incision; previous uterine rupture; previous full-thickness myomectomy; more than two previous caesarean sections in most guidelines; contracted pelvis. Relative contraindications include: placenta praevia, cord presentation, known foetal compromise, inter-delivery interval less than 18 months, and inability to perform emergency caesarean within 30 minutes. This is particularly important in the context of vbac calculatorulator calculations, where accuracy directly impacts decision-making. Professionals across multiple industries rely on precise vbac calculatorulator computations to validate assumptions, optimize processes, and ensure compliance with applicable standards. Understanding the underlying methodology helps users interpret results correctly and identify when additional analysis may be warranted.
Does the interval between caesarean and TOLAC matter?
Yes. A short inter-delivery interval of less than 18 months is associated with approximately 3-fold higher uterine rupture risk, likely due to incomplete scar healing. A minimum of 18-24 months between caesarean and TOLAC is recommended by most guidelines. Women with intervals under 18 months should be counselled about this increased risk.
What monitoring is required during TOLAC?
Continuous electronic fetal monitoring (CTG) is mandatory throughout active labour in TOLAC. IV access should be established. Theatre must be available within 30 minutes for emergency caesarean. The CTG should be reviewed frequently for decelerations, particularly prolonged decelerations or bradycardia that can indicate the early stages of uterine rupture. This is particularly important in the context of vbac calculatorulator calculations, where accuracy directly impacts decision-making. Professionals across multiple industries rely on precise vbac calculatorulator computations to validate assumptions, optimize processes, and ensure compliance with applicable standards. Understanding the underlying methodology helps users interpret results correctly and identify when additional analysis may be warranted.
Can women with two previous caesarean sections attempt VBAC?
TOLAC after two previous caesarean sections is associated with higher uterine rupture risk (~1.5%) and lower success rates (~70% in selected cases). It is not absolutely contraindicated in most guidelines but requires very careful individual assessment, detailed counselling, and senior obstetric involvement in the decision and intrapartum management. This is particularly important in the context of vbac calculatorulator calculations, where accuracy directly impacts decision-making. Professionals across multiple industries rely on precise vbac calculatorulator computations to validate assumptions, optimize processes, and ensure compliance with applicable standards. Understanding the underlying methodology helps users interpret results correctly and identify when additional analysis may be warranted.
专业提示
The single strongest predictor of VBAC success is a prior vaginal delivery — particularly a prior VBAC. When counselling a woman with a prior caesarean for malpresentation who has subsequently delivered vaginally, her VBAC probability is above 85% and the discussion should be framed positively around the very high likelihood of success, balanced against the small but real rupture risk.
你知道吗？
The VBAC rate in the United States plummeted from approximately 28% of eligible women in 1996 to just 9% in 2006 following high-profile malpractice cases and restrictive hospital policies. However, major obstetric bodies (ACOG, RCOG) subsequently published clear guidelines supporting TOLAC as a reasonable and safe option for appropriately selected women, leading to a gradual recovery of VBAC rates. This episode is a striking example of how medicolegal pressures can temporarily override clinical evidence in obstetric practice.
参考资料
- ›Grobman WA et al. Development of a nomogram for prediction of vaginal birth after cesarean delivery. Obstet Gynecol 2007.
- ›RCOG Green-top Guideline 45 — Birth After Previous Caesarean Birth. 2015.
- ›ACOG Practice Bulletin 205 — Vaginal Birth After Cesarean Delivery. Obstet Gynecol 2019.
- ›Guise JM et al. Uterine rupture in relation to the type of uterine scar: a systematic review. Lancet 2004.