{
  "id": "mare-gestation",
  "version": "7a914dddc92d",
  "status": "published",
  "name": "Mare Gestation Calculator",
  "question": "Mare gestation: when will she foal?",
  "summary": "Computes a mare’s foaling date from the last breeding date (340 days), the normal foaling window of 320 to 370 days, and how many days along she is.",
  "category": "biology",
  "subcategory": "animals",
  "url": "https://www.acalculator.org/biology/mare-gestation-calculator",
  "markdown": "https://www.acalculator.org/biology/mare-gestation-calculator.md",
  "kind": "function",
  "method": "Foaling date = last breeding date + 340 days; normal window = last breeding date + 320 to 370 days.",
  "assumptions": [
    "The estimate of 340 days is the top of Nebraska Extension’s average of 335 to 340 days; the window of 320 to 370 days is its normal range.",
    "Days count from the last breeding date as day 0. The window is wide: a foal within 320 to 370 days is normal."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "d": {
        "title": "Last breeding date",
        "description": "The last day the mare was bred (covered or inseminated).",
        "type": "string",
        "format": "date"
      },
      "on": {
        "title": "Today’s date",
        "description": "The day to count from, to show how far along the mare is.",
        "type": "string",
        "format": "date"
      }
    }
  },
  "outputs": {
    "due": {
      "label": "Estimated foaling date",
      "description": "The last breeding date + 340 days.",
      "format": "date"
    },
    "earliest": {
      "label": "Earliest normal foaling date",
      "description": "The last breeding date + 320 days.",
      "format": "date"
    },
    "latest": {
      "label": "Latest normal foaling date",
      "description": "The last breeding date + 370 days.",
      "format": "date"
    },
    "days": {
      "label": "Days along",
      "description": "Whole days from the last breeding date to today’s date.",
      "format": "integer"
    },
    "left": {
      "label": "Days to the estimated foaling date",
      "description": "Whole days from today’s date to the estimated foaling date; negative once it has passed.",
      "format": "integer"
    },
    "stage": {
      "label": "Where she is now",
      "description": "The stage on today’s date, in one line.",
      "format": "text"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "d": "2026-03-17",
      "on": "2026-10-03"
    },
    "outputs": {
      "due": "2027-02-20",
      "earliest": "2027-01-31",
      "latest": "2027-03-22",
      "days": 200,
      "left": 140,
      "stage": "Day 200 of about 340: 140 days to the estimated foaling date."
    },
    "text": "Your mare is due to foal about February 20, 2027, between January 31, 2027 and March 22, 2027."
  },
  "examples": [
    {
      "given": {
        "d": "2026-05-01",
        "on": "2026-12-01"
      },
      "expect": {
        "due": "2027-04-06",
        "earliest": "2027-03-17",
        "latest": "2027-05-06",
        "days": 214,
        "left": 126,
        "stage": "Day 214 of about 340: 126 days to the estimated foaling date."
      },
      "source": "Nebraska Extension, The Foaling Mare, G1874 (the average gestation length of mares is 335 to 340 days, but can range from 320 to 370 days), https://extensionpubs.unl.edu/publication/g1874/2008/html/view (retrieved 2026-10-03); hand calculation in content.mdx: 1 May 2026 + 320, 340 and 370 days"
    },
    {
      "given": {
        "d": "2027-04-10",
        "on": "2028-03-20"
      },
      "expect": {
        "due": "2028-03-15",
        "earliest": "2028-02-24",
        "latest": "2028-04-14",
        "days": 345,
        "left": -5
      },
      "source": "Nebraska Extension, The Foaling Mare, G1874 (the average gestation length of mares is 335 to 340 days, but can range from 320 to 370 days), https://extensionpubs.unl.edu/publication/g1874/2008/html/view (retrieved 2026-10-03); hand calculation in content.mdx (2028 is a leap year)"
    }
  ],
  "sources": [
    "Nebraska Extension, The Foaling Mare, G1874 (the average gestation length of mares is 335 to 340 days, but can range from 320 to 370 days). https://extensionpubs.unl.edu/publication/g1874/2008/html/view (retrieved 2026-10-03)",
    "Merck Veterinary Manual, Approximate Gestation Periods (horse: 11 months). https://www.merckvetmanual.com/multimedia/table/approximate-gestation-periods (retrieved 2026-10-03)"
  ],
  "related": [
    "goat-gestation",
    "sheep-gestation",
    "dog-pregnancy",
    "date"
  ],
  "changelog": []
}