{
  "id": "odds-of-twins",
  "version": "8c4c49f4dae5",
  "status": "published",
  "name": "Odds of Twins Calculator",
  "question": "What are my odds of having twins?",
  "summary": "Shows the chance that a birth is twins for a mother of your age, and of your race and Hispanic origin, from the US twin birth rates for 2018 (NCHS), as a percent and as 1 in N.",
  "category": "health",
  "subcategory": "pregnancy",
  "url": "https://www.acalculator.org/health/odds-of-twins-calculator",
  "markdown": "https://www.acalculator.org/health/odds-of-twins-calculator.md",
  "kind": "function",
  "method": "Chance = twin births per 1,000 births for the age group (or race and Hispanic origin group) ÷ 10, in percent; 1 in N = 1,000 ÷ rate.",
  "assumptions": [
    "US births in 2018; the rates include twins conceived with fertility treatment.",
    "The age rate and the group rate are separate published rates; they are not combined.",
    "A rate for all births is not a personal prediction for one pregnancy."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "age": {
        "title": "Mother’s age at birth",
        "description": "The mother’s age in years when the baby is born.",
        "type": "integer",
        "minimum": 12,
        "maximum": 55
      },
      "group": {
        "title": "Race and Hispanic origin",
        "description": "Optional: the mother’s race and Hispanic origin, for the rate of that group.",
        "type": "string",
        "enum": [
          "any",
          "white",
          "black",
          "hispanic"
        ]
      }
    }
  },
  "outputs": {
    "chance": {
      "label": "Chance of twins (by age)",
      "description": "Twin births per 1,000 births for mothers in this age group, as a percent.",
      "format": "percent"
    },
    "oneIn": {
      "label": "About 1 in",
      "description": "1,000 ÷ the age group’s rate, rounded.",
      "format": "integer"
    },
    "ageGroup": {
      "label": "Age group",
      "description": "The NCHS age group used.",
      "format": "text"
    },
    "groupChance": {
      "label": "Chance of twins (by race and Hispanic origin)",
      "description": "Twin births per 1,000 births for mothers of the chosen group, as a percent.",
      "format": "percent"
    },
    "groupOneIn": {
      "label": "About 1 in (by group)",
      "description": "1,000 ÷ the group’s rate, rounded.",
      "format": "integer"
    },
    "usChance": {
      "label": "US average",
      "description": "Twin births per 1,000 births for all US mothers in 2018 (32.6), as a percent.",
      "format": "percent"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "age": 32,
      "group": "any"
    },
    "outputs": {
      "chance": 3.62,
      "oneIn": 28,
      "ageGroup": "30 to 34",
      "usChance": 3.26
    },
    "text": "For a mother aged 32, about 3.62% of births are twins, about 1 in 28."
  },
  "examples": [
    {
      "given": {
        "age": 32,
        "group": "any"
      },
      "expect": {
        "chance": 3.62,
        "oneIn": 28,
        "ageGroup": "30 to 34",
        "usChance": 3.26
      },
      "source": "Martin JA, Osterman MJK, NCHS Data Brief No. 351, Is Twin Childbearing on the Decline? United States, 2014–2018 (twin birth rates per 1,000 births, 2018), https://www.cdc.gov/nchs/products/databriefs/db351.htm (retrieved 2026-10-05); hand calculation in content.mdx: 36.2 per 1,000 = 3.62%; 1,000 ÷ 36.2 = 27.6, about 1 in 28"
    },
    {
      "given": {
        "age": 41,
        "group": "black"
      },
      "expect": {
        "chance": 5.11,
        "oneIn": 20,
        "ageGroup": "40 and over",
        "groupChance": 4.05,
        "groupOneIn": 25
      },
      "source": "Martin JA, Osterman MJK, NCHS Data Brief No. 351, Is Twin Childbearing on the Decline? United States, 2014–2018 (twin birth rates per 1,000 births, 2018), https://www.cdc.gov/nchs/products/databriefs/db351.htm (retrieved 2026-10-05); hand calculation in content.mdx: 51.1 per 1,000 = 5.11%, 1,000 ÷ 51.1 = 19.6; 40.5 per 1,000 = 4.05%, 1,000 ÷ 40.5 = 24.7"
    },
    {
      "given": {
        "age": 19,
        "group": "hispanic"
      },
      "expect": {
        "chance": 1.71,
        "oneIn": 58,
        "ageGroup": "under 20",
        "groupChance": 2.44,
        "groupOneIn": 41
      },
      "source": "Martin JA, Osterman MJK, NCHS Data Brief No. 351, Is Twin Childbearing on the Decline? United States, 2014–2018 (twin birth rates per 1,000 births, 2018), https://www.cdc.gov/nchs/products/databriefs/db351.htm (retrieved 2026-10-05); hand calculation in content.mdx: 17.1 per 1,000 = 1.71%, 1,000 ÷ 17.1 = 58.5 (rounded half up, 58.48 gives 58); 24.4 per 1,000 = 2.44%, 1,000 ÷ 24.4 = 41.0"
    }
  ],
  "sources": [
    "Martin JA, Osterman MJK. Is Twin Childbearing on the Decline? United States, 2014–2018. NCHS Data Brief No. 351. National Center for Health Statistics, 2019, retrieved 2026-10-05. https://www.cdc.gov/nchs/products/databriefs/db351.htm"
  ],
  "related": [
    "pregnancy",
    "due-date",
    "ovulation"
  ],
  "changelog": []
}