{
    "name": "Pregnancy Conception Calculator",
    "url": "https://calculators.geekmidst.com/calculators/pregnancy-conception-calculator",
    "markdown": "https://calculators.geekmidst.com/calculators/pregnancy-conception-calculator.md",
    "category": "Health & fitness",
    "description": "Estimate the likely conception date and six-day fertile window from your due date or last period, based on 266 days from conception to due date.",
    "summary": "Conception usually happens around ovulation, about 266 days (38 weeks) before the due date. The fertile window is the six days ending on the day of ovulation.",
    "also_called": [
        "Conception date calculator",
        "When did I conceive calculator"
    ],
    "applies_to": "Worldwide",
    "last_updated": "2026-10-29",
    "computation": "Results are computed in the visitor’s browser on the HTML page. This API describes the calculator; it does not compute results.",
    "inputs": [
        {
            "name": "from",
            "label": "I know my",
            "type": "segmented",
            "default": "due",
            "options": {
                "due": "Due date",
                "lmp": "Last period"
            }
        },
        {
            "name": "due_date",
            "label": "Due date",
            "type": "date",
            "default": "2027-01-15",
            "shown_when": {
                "from": "due"
            }
        },
        {
            "name": "lmp",
            "label": "First day of last period",
            "type": "date",
            "default": "2026-04-10",
            "shown_when": {
                "from": "lmp"
            }
        },
        {
            "name": "cycle",
            "label": "Usual cycle length",
            "type": "number",
            "default": "28",
            "unit": "days",
            "optional": true,
            "shown_when": {
                "from": "lmp"
            }
        }
    ],
    "outputs": [
        {
            "key": "conception",
            "label": "Most likely conception date",
            "format": "text"
        },
        {
            "key": "fertile_window",
            "label": "Fertile window (6 days ending at ovulation)",
            "format": "text"
        },
        {
            "key": "due_date",
            "label": "Due date",
            "format": "text"
        },
        {
            "key": "lmp_equivalent",
            "label": "Pregnancy dated from (LMP equivalent)",
            "format": "text"
        }
    ],
    "formula": {
        "text": "Ovulation/conception = due date − 266 days (= cycle-adjusted LMP + 14 days);  Fertile window = ovulation − 5 days to ovulation",
        "variables": {
            "266 days": "38 weeks from conception to the due date",
            "fertile window": "the six days ending on ovulation (Wilcox et al., 1995)"
        }
    },
    "worked_example": {
        "inputs": {
            "from": "due",
            "due_date": "2027-01-15",
            "lmp": "2026-04-10",
            "cycle": "28"
        },
        "steps": [
            "Due date 15 January 2027 − 266 days = 24 April 2026 (estimated ovulation).",
            "Fertile window: 5 days before, 19 April, to 24 April 2026.",
            "Pregnancy is dated from 280 days before the due date: 10 April 2026."
        ],
        "result": "A due date of 15 January 2027 points to conception around 24 April 2026, from intercourse most likely between 19 and 24 April."
    },
    "interpretation": "The date is an estimate with a margin of several days either way: ovulation timing varies from cycle to cycle, and sperm can survive for days before ovulation. A due date set by an early ultrasound gives a better estimate than one from period dates.",
    "assumptions": [
        "Ovulation and conception 266 days before the due date (14 days after a cycle-adjusted LMP).",
        "Fertile window of six days ending on the day of ovulation (Wilcox, Weinberg and Baird, 1995)."
    ],
    "limitations": [
        "Cannot establish paternity or the exact day of intercourse.",
        "Less reliable with irregular cycles or a due date based only on period dates."
    ],
    "rules": null,
    "faqs": [
        {
            "question": "Can I find the exact day I conceived?",
            "answer": "Not from dates alone. Intercourse up to five days before ovulation can lead to pregnancy, so several days are equally plausible."
        },
        {
            "question": "Is conception the same as implantation?",
            "answer": "No. Implantation happens roughly a week after fertilisation; pregnancy tests turn positive after implantation."
        }
    ],
    "sources": [
        {
            "title": "Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med, 1995",
            "url": "https://carta.anthropogeny.org/libraries/bibliography/timing-sexual-intercourse-relation-ovulation-effects-probability-conception"
        },
        {
            "title": "ACOG Committee Opinion 700: Methods for Estimating the Due Date (2017)",
            "url": "https://www.healthcare.uiowa.edu/familymedicine/fpinfo/OB/OB2017/ACOG%20redating%20gestational%20age.pdf"
        }
    ],
    "related": [
        {
            "name": "Due Date Calculator",
            "url": "https://calculators.geekmidst.com/calculators/due-date-calculator"
        },
        {
            "name": "Pregnancy Calculator",
            "url": "https://calculators.geekmidst.com/calculators/pregnancy-calculator"
        },
        {
            "name": "Pregnancy Weight Gain Calculator",
            "url": "https://calculators.geekmidst.com/calculators/pregnancy-weight-gain-calculator"
        },
        {
            "name": "Date Difference Calculator",
            "url": "https://calculators.geekmidst.com/calculators/date-difference-calculator"
        }
    ],
    "disclaimer": "This is a general screening estimate for adults, not medical advice or a diagnosis. Talk to a qualified health professional about your health.",
    "publisher": "GM¹X CalcVerse"
}