{
  "id": "conception",
  "version": "f457f5f7ac05",
  "status": "published",
  "name": "Conception Calculator",
  "question": "When was my conception date?",
  "summary": "Estimates the day a pregnancy was conceived, and the days sex most likely happened, from the due date or from the first day of the last period and the cycle length.",
  "category": "health",
  "subcategory": "pregnancy",
  "url": "https://www.acalculator.org/health/conception-calculator",
  "markdown": "https://www.acalculator.org/health/conception-calculator.md",
  "kind": "function",
  "method": "From a due date: conception = due date − 266 days. From a last period: conception = last period + (cycle length − 14) days, due date = conception + 266 days. Likely window = conception − 5 days to conception.",
  "assumptions": [
    "The due date is 280 days after the first day of the last period, which assumes a 28-day cycle with ovulation on day 14 (ACOG).",
    "Ovulation comes 14 days before the next period would have started, so a longer cycle moves conception later.",
    "Conception is taken as the ovulation day. Sex up to 5 days before can lead to it, because sperm can survive that long (Wilcox and others, 1995).",
    "An ultrasound-dated due date is more accurate than one from the last period; the result is only as good as the date you enter. This is not medical advice."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "from": {
        "title": "What do you know?",
        "description": "Whether you know the due date or the first day of your last period.",
        "type": "string",
        "enum": [
          "due",
          "lmp"
        ]
      },
      "due": {
        "title": "Your due date",
        "description": "The estimated due date from your doctor or midwife.",
        "type": "string",
        "format": "date"
      },
      "lmp": {
        "title": "First day of your last period",
        "description": "The first day of bleeding of the last period before the pregnancy.",
        "type": "string",
        "format": "date"
      },
      "cycle": {
        "title": "Cycle length (days)",
        "description": "Days from the first day of one period to the first day of the next; 28 on average.",
        "type": "integer",
        "minimum": 20,
        "maximum": 45
      }
    }
  },
  "outputs": {
    "conception": {
      "label": "Likely conception date",
      "description": "The estimated day of ovulation and conception.",
      "format": "date"
    },
    "windowStart": {
      "label": "Sex most likely from",
      "description": "Five days before the conception date; the likely window ends on the conception date.",
      "format": "date"
    },
    "dueDate": {
      "label": "Due date",
      "description": "The due date: 266 days after conception, or the date you entered.",
      "format": "date"
    },
    "periodStart": {
      "label": "Last period started",
      "description": "The first day of the last period: the date you entered, or 280 days before the due date.",
      "format": "date"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "from": "due",
      "due": "2027-02-16",
      "lmp": "2026-07-21",
      "cycle": 28
    },
    "outputs": {
      "conception": "2026-05-26",
      "windowStart": "2026-05-21",
      "dueDate": "2027-02-16",
      "periodStart": "2026-05-12"
    },
    "text": "You most likely conceived around May 26, 2026, from sex between May 21, 2026 and May 26, 2026."
  },
  "examples": [
    {
      "given": {
        "from": "due",
        "due": "2027-03-01"
      },
      "expect": {
        "conception": "2026-06-08",
        "windowStart": "2026-06-03",
        "periodStart": "2026-05-25",
        "dueDate": "2027-03-01"
      },
      "source": "ACOG Committee Opinion 700 (due date = last period + 280 days, ovulation on day 14); hand calculation in content.mdx: March 1, 2027 − 266 days"
    },
    {
      "given": {
        "from": "lmp",
        "lmp": "2026-06-01",
        "cycle": 28
      },
      "expect": {
        "conception": "2026-06-15",
        "windowStart": "2026-06-10",
        "dueDate": "2027-03-08"
      },
      "source": "ACOG Committee Opinion 700: a 28-day cycle ovulates on day 14 and the due date is 280 days after the last period; Wilcox and others 1995 (6-day window)"
    },
    {
      "given": {
        "from": "lmp",
        "lmp": "2026-06-01",
        "cycle": 35
      },
      "expect": {
        "conception": "2026-06-22",
        "windowStart": "2026-06-17",
        "dueDate": "2027-03-15"
      },
      "source": "hand calculation in content.mdx: ovulation 14 days before the next period (ACOG FAQ024), so day 35 − 14 = 21 after June 1"
    }
  ],
  "sources": [
    "American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics and Gynecology 2017;129:e150–e154. doi:10.1097/aog.0000000000002046",
    "American College of Obstetricians and Gynecologists, Fertility Awareness-Based Methods of Family Planning (FAQ024, last reviewed February 2025). https://www.acog.org/womens-health/faqs/fertility-awareness-based-methods-of-family-planning",
    "Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine 1995;333:1517–1521. doi:10.1056/nejm199512073332301"
  ],
  "related": [
    "period",
    "pregnancy-test",
    "ivf-due-date"
  ],
  "changelog": [
    {
      "date": "2026-09-28",
      "note": "The due date box is labelled Your due date, apart from the Due date choice."
    }
  ]
}