Caesarean Birth and Vaginal Birth After Caesarean Section (VBAC/TOLAC)

Definition, epidemiology and clinical significance

Cesarean delivery is one of the most frequently performed major surgical procedures worldwide, and its rate has risen steadily over the past four decades. Unless the total number of births declines, the strongest leverage for limiting the cesarean rate at the population level comes down to two strategies: (1) preventing unnecessary primary (first) cesarean deliveries and (2) supporting trial of labor after cesarean (TOLAC) in suitable candidates. These two strategies form the shared framework of the 2014 ACOG/SMFM Obstetric Care Consensus No. 1 "Safe Prevention of the Primary Cesarean Delivery" and the ACOG Practice Bulletin No. 205 "Vaginal Birth After Cesarean Delivery" (2019) guideline (ACOG/SMFM OCC No. 1, 2014; ACOG PB 205, 2019).

Terminology should be clarified: TOLAC (trial of labor after cesarean) is a trial of planned vaginal birth for a pregnant woman who has had a previous cesarean section; Whatever the outcome, it defines the initiative. VBAC (vaginal birth after cesarean) is the successful completion of this attempt with a vaginal birth. PRCB (planned repeat cesarean birth) is a planned repeat cesarean section performed before the labor begins. The frequency of TOLAC in the USA was reported as 21.7% in 2020, approximately 75% of these trials were successful, and the overall VBAC rate was 16.2% (UpToDate, "Choosing the route of delivery after cesarean birth", 2025).

Indications for primary caesarean section and safe reduction

The most common indications for primary cesarean section, in order of frequency, are: labor dystocia (most common), alarming/uncertain fetal heart rate [FHR] tracing, fetal malpresentation, multiple pregnancy, and suspected fetal macrosomia (ACOG/SMFM OCC No. 1, 2014). The greatest reduction potential is in the first two headings; Therefore, current guidelines recommend keeping the "dystocia" threshold late by adopting contemporary norms for labor duration.

Definitions of labor duration and arrest criteria

Contemporary data (Consortium on Safe Labor / Zhang curves) have shown that the latent phase begins slowly and the active phase begins after 6 cm of dilation. Therefore, a diagnosis of "active phase arrest" should not be made before 6 cm.

• Latent phase: Prolonged latent phase alone (nulliparous >20 hours, multiparous >14 hours) is not an indication for cesarean section; can be expected as long as the maternal/fetal status is reassuring (ACOG/SMFM OCC No. 1, 2014).
• Active phase protraction: at ≥6 cm dilatation, <1–2 cm dilatation progression in 2 hours (UpToDate, "Labor: abnormal first stage", 2026).
• Active phase arrest: Membranes open + dilatation ≥6 cm + either — absence of cervical change for ≥4 hours despite adequate contraction (≥200 Montevideo units), or absence of change despite administration of oxytocin for ≥6 hours with inadequate contraction. These periods, rather than the historical 2-hour threshold, have been found to be safe for the mother and fetus and increase the chances of vaginal birth (ACOG/SMFM OCC No. 1, 2014; UpToDate 2026).

Prolonged second stage and failed induction

There is no single absolute upper limit to the duration of the second phase; the decision is individualized based on maternal-fetal status, progression, and patient preference (ACOG/SMFM clinical guideline).

Conservative measures in the management of worrisome/uncertain FHR tracing, such as amnioinfusion (in case of recurrent variable decelerations), maternal position change and tocolysis in tachysystole, help to avoid urgent caesarean section in category II trace. Suspicion of macrosomia alone does not constitute an indication for cesarean section unless the weight reaches the threshold of ≥5000 g (without diabetes) or ≥4500 g (with diabetes) (ACOG/SMFM OCC No. 1, 2014).

Pathophysiology: scar healing and rupture mechanics

The biomechanical strength of the previous hysterotomy scar depends on the quality of myometrial wound healing. In a systematic review of human and animal studies, the histology of uterine scars is characterized by disorganized smooth muscle, collagen fibrous fibrosis, and few endometrial glands; Collagen fiber orientation has been modeled as the most important factor for the biomechanical properties of the scarred uterus (UpToDate, "Uterine rupture…", 2024). Rupture is a life-threatening condition that results in a noticeable change in maternal/fetal status with complete separation of all uterine layers, including the serosa. Dehiscence (uterine window) is a clinically silent, incomplete dehiscence; It is usually detected incidentally during a repeat cesarean section and does not cause serious consequences. The interchangeable use of the two terms in the literature is the main reason for the variability in rupture rates.

TOLAC candidate selection

The ideal candidate is a pregnant woman with a high likelihood of VBAC and a very low likelihood of intrapartum uterine rupture. Because the risk of rupture largely depends on the type of previous hysterotomy, it is essential to obtain reports of previous cesarean sections and document the location and extent of the incision (ACOG PB 205, 2019; UpToDate 2025).

Suitable and potential candidates

• Single previous lower segment transverse (low transverse) incision: The group with the lowest risk of scar dehiscence. VBAC rate 60–80%, estimated risk of rupture 0.4–0.7% (UpToDate 2025; NICHD consensus).
• Two previous lower segment transverse incisions: acceptable candidate according to ACOG; the probability of success is similar to the previous one, but the risk of rupture is slightly higher (1 vs 2 previous caesarean sections in a meta-analysis: 0.72% vs 1.59%; OR 0.42) (ACOG PB 205, 2019).
• Unknown incision type: Since most pregnancies have a lower segment transverse incision, TOLAC is considered reasonable unless there is a high suspicion of a classic incision (e.g. cesarean at <~28 weeks, known lower segment pathology); In MFMU data, rupture is 0.5% in the unknown scar and 0.7% in the known lower segment (ACOG PB 205, 2019).
• Twin pregnancy, suspected macrosomia, obesity, diabetes, >40 weeks: Not a contraindication per se; TOLAC may be offered with individualized counseling. Pooled success in twin ~72%, rupture ~0.87% (UpToDate 2025).

Contraindications (situations favoring planned repeat cesarean birth [PRCB])

Due to the high risk of intrapartum rupture, TOLAC is not recommended for the following pregnant women (ACOG PB 205, 2019):

• Classic, T- or J-incision (transfundal/upper segment): The frequency of rupture in classical or T incision is 4–9% in the literature review; lower segment >5 times transverse. Rupture increased to 10.6% in the group that went into labor after classic cesarean section (0.3% in those who did not go into labor).
• Previous uterine rupture: The risk of re-rupture in a subsequent pregnancy was found to be 10% in a systematic review (after complete rupture); not recommended.
• History of transmyometrial incision for open fetal surgery and extensive myomectomy entering the uterine cavity: risk of rupture unacceptably high.
• Standard contraindications to vaginal birth (e.g. placenta previa).

Additionally, a short interpregnancy interval (<6 months) may increase the risk of rupture to ~2–3%; this should be included in decision sharing (UpToDate 2025).

VBAC success predictor and calculator

The individual probability of success can be estimated with a calculator at the time of entry into prenatal care. The MFMU Network calculator has been used for many years; however, the original model assigned black and Hispanic patients an average of 5–15 points lower odds than white patients with the same clinical characteristics. Therefore, an updated calculator that excludes race and ethnicity was released in 2021 (Grobman et al., MFMU, May 2021). The current model uses six variables: age, height, weight, obstetric history (previous vaginal birth/prior VBAC), recurrent indication of previous cesarean section (e.g. failure to progress), and hypertension. ACOG recommends not using the older version that includes race/ethnicity (ACOG Practice Advisory, December 2021).

The two factors that most strongly increase success are a history of previous VBAC and previous vaginal birth (each increasing the likelihood of VBAC by at least threefold). The factor most strongly associated with failure (intrapartum cesarean section) is that the previous cesarean section was performed for recurrent indication (e.g. failure to progress) (UpToDate 2025).

Uterine rupture risk: numerical assessment

This is the issue at the heart of the TOLAC consultation and must be conveyed in accurate numbers. The risk of uterine rupture in a pregnant woman with a single previous lower segment transverse incision is approximately 0.5–0.9% (UpToDate, "TOLAC: Intrapartum management", 2024). The NICHD consensus predicted ~468 ruptures per 100,000 pregnant women undergoing TOLAC at any given week of gestation (approximately 1 in every 200 TOLACs). The overall incidence of rupture in pregnant women with a previous cesarean section is ~0.3% (300/100,000) regardless of mode of delivery (UpToDate 2024).

Labor and induction increase risk (EP—conflicting evidence transparently reported): In a large prospective MFMU study, the frequency of rupture in pregnant women with one or more cesareans differed markedly by labor status (absolute differences are small, ≤1%):

Important nuances: (1) The clinically relevant comparator is expectant management, not spontaneous labor; in this comparison, induction has not been consistently shown to increase rupture. (2) Induction does not appear to increase the risk of rupture in pregnant women with a prior vaginal birth and/or a favorable cervix (Bishop ≥6) (1.5% with induction vs. 0.8% spontaneous without prior vaginal birth; 0.6% vs. 0.4% with prior vaginal birth) (UpToDate 2025). (3) Misoprostol (PGE1) is contraindicated—one randomized trial was stopped early for safety concerns, and ACOG recommends against misoprostol in term pregnant women with prior uterine incisions. Vaginal PGE2 is not universally prohibited, though UpToDate authors prefer mechanical methods (ACOG PB 205, 2019; UpToDate 2025).

Intrapartum monitoring and management

Management of labor in TOLAC is largely the same as in the scarless uterus; differences arise from the risk of rupture (UpToDate, "TOLAC: Intrapartum management", 2024):

• Facility resources: ACOG recommends that TOLAC be performed in facilities that can perform emergency cesarean section; TOLAC is not recommended for planned home birth.
• Consent: Consent for both TOLAC and repeat cesarean section is obtained at admission; The risk of rupture and success rate are documented.
• Continuous cardiotocography: The most common (~70%) and often immediate sign of rupture is abnormal FHR (most commonly fetal bradycardia); Continuous FHR and contraction monitoring is routinely recommended.
• Laboratory/access: Basal hemoglobin/hematocrit + blood type and antibody screening; intravenous access. TOLAC transfusion risk is ~2%.
• Neuraxial analgesia: Does not mask signs of rupture, does not reduce the likelihood of VBAC, and allows rapid delivery in emergency cesarean section; can be recommended. Pain after an effective epidural or the need for frequent doses may be warnings of rupture.
• Oxytocin: Augmentation and induction are acceptable. There is no high evidence for the optimal protocol; some authors prefer a low-dose regimen (initial 0.5–2 milliunits/min, increase by 1–2 milliunits/min every 15–40 min). Most units notify physicians at 20 milliunits/min; The possibility of confounding (dystocia) should be taken into consideration when increasing doses.

As the second stage lengthens, both the probability of vaginal delivery decreases and the frequency of rupture increases: in one observational study, the frequency of rupture/dehiscence increased from 0.7% at <1 hour to ~3% at ≥3 hours. Therefore, the threshold for operative delivery should be kept low when rupture is suspected (UpToDate 2024).

Rupture: diagnosis and emergency management

Signs of uterine rupture, in order of frequency: sudden-onset category II/III FHR tracing (most commonly bradycardia), abdominal pain (which may be masked by neuraxial analgesia), vaginal bleeding (which may be scant despite major intra-abdominal hemorrhage), loss of station of the presenting part, hematuria, hemodynamic instability, and contraction pattern changes ("stair-step" sign). Definitive diagnosis is established by demonstrating disruption of all uterine layers on imaging or at laparotomy. Management is emergency cesarean delivery, with hemodynamic stabilization (fluids/blood products), immediate notification of the anesthesia and neonatology teams, and a midline incision if hemoperitoneum is suspected. The choice between defect repair and hysterectomy depends on hemorrhage control, desire for future fertility, and surgical expertise; 14–33% of ruptures require hysterectomy (UpToDate 2024).

Morbidity of repeat cesarean section and the spectrum of placenta accreta

One of the strongest rationales to support VBAC is the increased risk of placenta accreta spectrum (PAS) with increasing numbers of repeat cesareans. In the Landmark MFMU study (Silver et al., 2006; 30,132 pregnant women), the frequency of PAS increased dramatically with the number of caesarean sections:

The increasing number of cesarean sections also increases the risk of placenta previa, peripartum hysterectomy, bladder/bowel injury, transfusion, and the need for intensive care. Therefore, successful VBAC in pregnant women planning multiple pregnancies in the future is particularly valuable in avoiding cumulative surgical morbidity (UpToDate 2025; ACOG PB 205, 2019). In the TOLAC versus PRCB comparison, propensity analysis estimated that ~62 pregnancies needed to undergo PRCB to prevent one maternal adverse outcome and ~43 pregnant women needed to undergo PRCB to prevent one neonatal adverse outcome (UpToDate 2025).

Controversial points and current changes

• Induction vs. expectant management: Whether induction increases uterine rupture becomes less clear when the comparator is expectant management rather than spontaneous labor. Two of three observational studies showed no difference, while one reported an increased risk (1.4% vs. 0.5%) (UpToDate 2025)—this uncertainty should be communicated transparently and the decision individualized.
• Vaginal PGE2: Although misoprostol is strictly contraindicated, the evidence for PGE2 is conflicting; In the 2021 meta-analysis, rupture with PGE2 was found to be ~5/1000 (single lower segment transverse). ACOG does not address PGE2; mechanical methods are preferred (UpToDate 2025).
• Lower uterine segment thickness measurement: Routine is not recommended as no threshold value reliably predicts rupture; In the randomized study (≤3.5 mm threshold) the outcome difference was not significant (UpToDate 2024).
• Single vs double layer closure: Single layer closure of the previous hysterotomy is not routinely taken into account in TOLAC counseling; Observational data have not shown an association with rupture, but most studies are underpowered (UpToDate 2025).

Clinical summary

• Two pillars of reducing the cesarean rate: preventing unnecessary primary cesareans (contemporary labor thresholds—Table 1) and promoting TOLAC in suitable candidates.
• Active phase arrest is recognized by ≥6 cm + ROM + (absence of change for ≥4 h with adequate contraction or ≥6 h with oxytocin); Latent phase prolongation alone is not an indication for cesarean section.
• VBAC success rate in single previous lower segment transverse incision is 60–80%; risk of uterine rupture 0.5–0.9% (lower limit 0.4–0.7%).
• Induction/oxytocin slightly increases risk (spontaneous ~4/1000 → oxytocin ~11/1000 → prostaglandin ~14/1000); misoprostol is contraindicated. The risk of induction is not significantly increased in those with a previous vaginal birth/favorable cervix.
• Absolute contraindications: classic/T/J incision, previous uterine rupture, transfundal surgery.
• The race/ethnicity-free 2021 MFMU calculator should be used to predict success.
• Continuous FHR monitoring is mandatory; the most common sign of uterine rupture is sudden fetal bradycardia; the facility must be immediately ready for emergency cesarean delivery.
• Increased repeat cesarean section exponentially increases the risk of PAS and previa (Silver 2006 — Table 3); this supports the long-term value of successful VBAC.

🔄 Current Developments (2026)

During 2024-2026, ACOG PB 205 (2019), RCOG Green-top 45 (2015), and ACOG-SMFM Obstetric Care Consensus 1 (2014) remain in effect; There was no number or version change. With this, FIGO published the first global set of recommendations in this field, accumulating new actionable evidence on the oxytocin protocol and prediction models. The topics that are not included in the current guide are summarized below.

New thresholds for action (FIGO 2025)

• Timing of induction: If indicated, induction should be performed at 39–41 weeks; Induction at ≥41 weeks increases the likelihood of rupture, while the 39-41 week window provides lower rates of cesarean section and higher VBAC success (Rec 22).
• Interpartum interval: At least 18 months is recommended; <18 months increases the risk of rupture (OR 3.0; 95% CI 1.3-7.2) (Rec 11). A more useful threshold for counseling than the <6 months data in the current guideline.
• Rupture by incision type: Low-segment transverse 0.7%; low vertical 2%; classical/inverted-T/J 1.9% (4–9% if extending into the contractile upper segment); unknown type 0.5%. FIGO does not consider a low-vertical incision confined to the non-contractile segment to be contraindicated and considers VBAC safe (Rec 8)—a distinction not addressed as a separate category in the current guideline.
• Induction method: Mechanical methods (amniotomy + transcervical Foley) are preferred to prostaglandins (Rec 23); misoprostol is not used in term pregnancy due to high risk of rupture (Rec 24) — confirmation at global guideline level, in line with current guidance.

FIGO also states that it does not use the term "TOLAC" due to its connotation of "trial/trial" upon feedback from women; It is a terminology stance that can be taken into consideration in patient communication (FIGO 2025, PMID 40728304).

Conflicting stances of authority

Two previous cesarean sections: ACOG PB 205 considers these patients acceptable candidates for VBAC, while FIGO 2025 is more cautious—generally not recommending it, only considering the patient with a previous vaginal birth and a strong willingness in a high-resource center with immediate access to emergency cesarean (success ~69.5-74.6%, rupture ~1.36-1.8%, but major maternal morbidity aOR 1.61 [1.11-2.33]) (Rec 6).

Prediction models: External validation series show that the 2021 race-neutral MFMU calculator is of superior caliber (particularly in black/Hispanic patients) than the 2007 version (J Midwifery Womens Health 2025). On the other hand, FIGO 2025 finds the external validation of most of the models weak and does not recommend their routine use (Rec 14). While ACOG/UpToDate 2021 says use the calculator, FIGO says don't use the models routinely.

Points confirmed (not changed)

FIGO does not recommend routine antenatal lower uterine segment thickness measurement as no single valid threshold has been identified (Rec 13)—in line with current guidance. The 2024 meta-analysis (PMC10869045) confirms that in TOLAC vs elective repeat cesarean section, the risk of rupture is significantly higher (OR 2.01 [1.48-2.74]) but the absolute risk is low, and TOLAC remains a valid alternative in the suitable candidate. NICE NG192 (caesarean section) updated 10 June 2025 (oxytocin recommendation linked to carbetocin); It does not change VBAC thresholds but indicates that the guide should be kept active. The ACNM 2024 clinical bulletin promotes ACOG-compliant hospital-based VBAC care from an interdisciplinary perspective.

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