{
  "id": "vbac",
  "version": "8b96e8e93e89",
  "status": "published",
  "name": "VBAC Calculator",
  "question": "What is my chance of a VBAC?",
  "summary": "Estimates the chance of a vaginal birth after cesarean (VBAC) with a trial of labor from the MFMU Network model without race and ethnicity (Grobman 2021).",
  "category": "health",
  "subcategory": "clinical",
  "url": "https://www.acalculator.org/health/vbac-calculator",
  "markdown": "https://www.acalculator.org/health/vbac-calculator.md",
  "kind": "function",
  "method": "Chance (%) = 100 × e^w ÷ (1 + e^w), with w = −5.952 − 0.023 × age − 0.024 × weight (kg) + 0.056 × height (cm) − 0.597 × arrest + 0.868 × vaginal birth only before the cesarean + 1.869 × earlier VBAC − 0.966 × treated chronic hypertension.",
  "assumptions": [
    "For people with one earlier low-transverse cesarean, one baby head-down, at 37 weeks or later, who have a trial of labor.",
    "The equation uses the coefficients printed in Grobman 2021, so results can differ from the paper’s own table by a few tenths of a percent.",
    "The chance is rounded to 1 decimal, halves up."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "age": {
        "title": "Your age",
        "description": "Your age in whole years.",
        "type": "integer",
        "minimum": 16,
        "maximum": 50
      },
      "height": {
        "title": "Height",
        "description": "Your height.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 1.4,
        "maximum": 1.95
      },
      "weight": {
        "title": "Weight before pregnancy",
        "description": "Your weight before this pregnancy.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "mass",
        "minimum": 35,
        "maximum": 180
      },
      "vag": {
        "title": "Earlier vaginal birth",
        "description": "Any vaginal birth before this pregnancy: none, only before the cesarean, or a VBAC (after it).",
        "type": "string",
        "enum": [
          "none",
          "before",
          "vbac"
        ]
      },
      "arrest": {
        "title": "Cesarean for labor that stopped?",
        "description": "Whether the cesarean was done because labor stopped progressing (an arrest of dilation or descent).",
        "type": "string",
        "enum": [
          "no",
          "yes"
        ]
      },
      "htn": {
        "title": "Treated chronic high blood pressure?",
        "description": "Whether you have high blood pressure from before pregnancy that is treated with medicine.",
        "type": "string",
        "enum": [
          "no",
          "yes"
        ]
      }
    }
  },
  "outputs": {
    "chance": {
      "label": "Chance of VBAC",
      "description": "The predicted chance of a vaginal birth with a trial of labor, as a percent, to 1 decimal.",
      "format": "percent"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "vag": "none",
      "arrest": "no",
      "htn": "no",
      "age": 30,
      "height": 1.56,
      "weight": 71
    },
    "outputs": {
      "chance": 59.6
    },
    "text": "The predicted chance of a vaginal birth after cesarean is 59.6%."
  },
  "examples": [
    {
      "given": {
        "age": 28,
        "weight": 68.0388555,
        "height": 1.6256,
        "vag": "before",
        "arrest": "no",
        "htn": "no"
      },
      "expect": {
        "chance": 85.1
      },
      "source": "Grobman 2021 equation: 68.039 kg, 162.56 cm; w = −5.952 − 0.644 − 1.633 + 9.103 + 0.868 = 1.7424; 85.10% (hand calculation in content.mdx)"
    },
    {
      "given": {
        "age": 30,
        "weight": 71,
        "height": 1.71,
        "vag": "vbac",
        "arrest": "no",
        "htn": "no"
      },
      "expect": {
        "chance": 95.7
      },
      "source": "Grobman 2021, equation in Results: w = 3.0955, 95.69%; the paper’s Table 3 example 1 shows 95.6 from unrounded coefficients (hand calculation in content.mdx)"
    },
    {
      "given": {
        "age": 30,
        "weight": 71,
        "height": 1.56,
        "vag": "before",
        "arrest": "no",
        "htn": "no"
      },
      "expect": {
        "chance": 77.9
      },
      "source": "Grobman 2021 equation: 77.87%; Table 3 example 3 shows 77.7 (hand calculation in content.mdx)"
    },
    {
      "given": {
        "age": 30,
        "weight": 71,
        "height": 1.56,
        "vag": "none",
        "arrest": "no",
        "htn": "no"
      },
      "expect": {
        "chance": 59.6
      },
      "source": "Grobman 2021 equation: 59.63%; Table 3 example 7 shows 59.4 (hand calculation in content.mdx)"
    },
    {
      "given": {
        "age": 30,
        "weight": 71,
        "height": 1.56,
        "vag": "none",
        "arrest": "yes",
        "htn": "no"
      },
      "expect": {
        "chance": 44.8
      },
      "source": "Grobman 2021 equation: 44.84%; Table 3 example 10 shows 44.6 (hand calculation in content.mdx)"
    },
    {
      "given": {
        "age": 37,
        "weight": 71,
        "height": 1.56,
        "vag": "none",
        "arrest": "no",
        "htn": "yes"
      },
      "expect": {
        "chance": 32.4
      },
      "source": "Grobman 2021 equation: 32.37%; Table 3 example 12 shows 32.1 (hand calculation in content.mdx)"
    }
  ],
  "sources": [
    "Grobman WA, Sandoval G, Rice MM, et al. Prediction of vaginal birth after cesarean delivery in term gestations: a calculator without race and ethnicity. Am J Obstet Gynecol. 2021;225(6):664.e1-664.e7. https://doi.org/10.1016/j.ajog.2021.05.021",
    "NICHD Maternal-Fetal Medicine Units Network. Vaginal Birth After Cesarean Calculator. https://mfmunetwork.bsc.gwu.edu/web/mfmunetwork/vaginal-birth-after-cesarean-calculator (retrieved 2026-09-29)"
  ],
  "related": [],
  "changelog": [
    {
      "date": "2026-09-30",
      "note": "Published after this site's editorial review against the cited sources (no clinician review); the page notices say so."
    },
    {
      "date": "2026-09-30",
      "note": "The answer sentence prints one percent sign, not two."
    }
  ]
}