{
  "id": "fertility",
  "version": "bad4716269bf",
  "status": "published",
  "name": "Fertility Calculator",
  "question": "When is my fertility window?",
  "summary": "Finds the fertile days of a cycle from the first day of the last period and the shortest and longest recent cycles: from 5 days before the earliest likely ovulation to 1 day after the latest (ACOG).",
  "category": "health",
  "subcategory": "pregnancy",
  "url": "https://www.acalculator.org/health/fertility-calculator",
  "markdown": "https://www.acalculator.org/health/fertility-calculator.md",
  "kind": "function",
  "method": "Cycle day 1 is the first day of the last period. Ovulation falls on day (cycle length − 13), 14 days before the next period; the fertile window runs from day (shortest − 18) to day (longest − 12).",
  "assumptions": [
    "Ovulation happens about 14 days before the next period starts (ACOG).",
    "Pregnancy is possible from 5 days before ovulation to 1 day after it: sperm live up to about 5 days and the egg about 1 day.",
    "Your next cycle is no shorter than your shortest and no longer than your longest recent cycle.",
    "This is an estimate for planning, not medical advice and not a way to prevent pregnancy."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "lmp": {
        "title": "First day of your last period",
        "description": "The first day of bleeding of your most recent period: day 1 of this cycle.",
        "type": "string",
        "format": "date"
      },
      "short": {
        "title": "Shortest cycle (days)",
        "description": "Your shortest cycle over the last 6 or more: days from the first day of one period to the first day of the next.",
        "type": "integer",
        "minimum": 21,
        "maximum": 45
      },
      "long": {
        "title": "Longest cycle (days)",
        "description": "Your longest cycle over the same months. Type the same number as the shortest if your cycles are regular.",
        "type": "integer",
        "minimum": 21,
        "maximum": 45
      }
    }
  },
  "outputs": {
    "fertileStart": {
      "label": "Fertile from",
      "description": "The first fertile day: 5 days before the earliest likely ovulation.",
      "format": "date-long"
    },
    "fertileEnd": {
      "label": "Fertile until",
      "description": "The last fertile day: 1 day after the latest likely ovulation.",
      "format": "date-long"
    },
    "firstDay": {
      "label": "First fertile cycle day",
      "description": "The first fertile day as a day of the cycle (day 1 is the first day of the period): shortest − 18.",
      "format": "integer"
    },
    "lastDay": {
      "label": "Last fertile cycle day",
      "description": "The last fertile day as a day of the cycle: longest − 12.",
      "format": "integer"
    },
    "days": {
      "label": "Fertile days",
      "description": "How many days the window spans: longest − shortest + 7.",
      "format": "integer"
    },
    "ovulationFrom": {
      "label": "Likely ovulation from",
      "description": "The earliest likely ovulation: 14 days before a period after the shortest cycle.",
      "format": "date"
    },
    "ovulationTo": {
      "label": "Likely ovulation to",
      "description": "The latest likely ovulation: 14 days before a period after the longest cycle.",
      "format": "date"
    },
    "nextFrom": {
      "label": "Next period from",
      "description": "The earliest start of the next period: the last period plus the shortest cycle.",
      "format": "date"
    },
    "nextTo": {
      "label": "Next period by",
      "description": "The latest start of the next period: the last period plus the longest cycle.",
      "format": "date"
    },
    "standard": {
      "label": "Standard Days method",
      "description": "Whether the Standard Days method (days 8 to 19) fits your cycles, and its dates when it does.",
      "format": "text"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "lmp": "2026-09-25",
      "short": 28,
      "long": 28
    },
    "outputs": {
      "fertileStart": "2026-10-04",
      "fertileEnd": "2026-10-10",
      "firstDay": 10,
      "lastDay": 16,
      "days": 7,
      "ovulationFrom": "2026-10-09",
      "ovulationTo": "2026-10-09",
      "nextFrom": "2026-10-23",
      "nextTo": "2026-10-23",
      "standard": "Your cycles are 26 to 32 days, so the Standard Days method also applies: days 8 to 19, October 2, 2026 to October 13, 2026."
    },
    "text": "Your fertile window is Sunday, October 4, 2026 to Saturday, October 10, 2026, cycle days 10 to 16."
  },
  "examples": [
    {
      "given": {
        "lmp": "2026-09-01",
        "short": 28,
        "long": 28
      },
      "expect": {
        "fertileStart": "2026-09-10",
        "fertileEnd": "2026-09-16",
        "firstDay": 10,
        "lastDay": 16,
        "days": 7,
        "ovulationFrom": "2026-09-15",
        "nextFrom": "2026-09-29",
        "standard": "Your cycles are 26 to 32 days, so the Standard Days method also applies: days 8 to 19, September 8, 2026 to September 19, 2026."
      },
      "source": "hand calculation in content.mdx: ovulation on day 28 − 13 = 15 (September 15); ACOG FAQ024, Fertility Awareness-Based Methods of Family Planning: ovulation about 14 days before the next period; pregnancy possible from 5 days before to 1 day after ovulation; Standard Days method days 8–19 for 26- to 32-day cycles (https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning, retrieved 2026-10-02)"
    },
    {
      "given": {
        "lmp": "2026-02-10",
        "short": 26,
        "long": 32
      },
      "expect": {
        "fertileStart": "2026-02-17",
        "fertileEnd": "2026-03-01",
        "firstDay": 8,
        "lastDay": 20,
        "days": 13,
        "ovulationFrom": "2026-02-22",
        "ovulationTo": "2026-02-28",
        "nextFrom": "2026-03-08",
        "nextTo": "2026-03-14"
      },
      "source": "hand calculation in content.mdx: days 26 − 18 = 8 to 32 − 12 = 20; ACOG FAQ024, Fertility Awareness-Based Methods of Family Planning: ovulation about 14 days before the next period; pregnancy possible from 5 days before to 1 day after ovulation; Standard Days method days 8–19 for 26- to 32-day cycles (https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning, retrieved 2026-10-02)"
    },
    {
      "given": {
        "lmp": "2028-02-20",
        "short": 24,
        "long": 35
      },
      "expect": {
        "fertileStart": "2028-02-25",
        "fertileEnd": "2028-03-13",
        "days": 18,
        "ovulationFrom": "2028-03-01",
        "ovulationTo": "2028-03-12",
        "standard": "The Standard Days method (days 8 to 19) is only for cycles that are always 26 to 32 days long."
      },
      "source": "hand calculation in content.mdx: days 6 to 23, with February 29, 2028; ACOG FAQ024, Fertility Awareness-Based Methods of Family Planning: ovulation about 14 days before the next period; pregnancy possible from 5 days before to 1 day after ovulation; Standard Days method days 8–19 for 26- to 32-day cycles (https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning, retrieved 2026-10-02)"
    }
  ],
  "sources": [
    "American College of Obstetricians and Gynecologists, FAQ024 Fertility Awareness-Based Methods of Family Planning (ovulation about 14 days before the next period; pregnancy possible from 5 days before to 1 day after ovulation; sperm live about 3 and up to 5 days, the egg about 24 hours; Standard Days method days 8–19 for cycles of 26 to 32 days; 12–24 of 100 pregnant in the first year of typical use), retrieved 2026-10-02. https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning"
  ],
  "related": [
    "ovulation",
    "period",
    "conception",
    "due-date"
  ],
  "changelog": []
}