{
    "name": "Conception Calculator",
    "url": "https://calculators.geekmidst.com/calculators/conception-calculator",
    "markdown": "https://calculators.geekmidst.com/calculators/conception-calculator.md",
    "category": "Health & fitness",
    "description": "Estimate the likely conception date and the window of intercourse that led to it, from your due date or from your last period and cycle length.",
    "summary": "A pregnancy lasts about 266 days from conception, or 280 days from the first day of the last period. Counting back from a due date, or forward from a period, estimates when conception happened.",
    "also_called": [
        "Conception date 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": "mode",
            "label": "I know my",
            "type": "segmented",
            "default": "due",
            "options": {
                "due": "Due date",
                "lmp": "Last period"
            }
        },
        {
            "name": "due_date",
            "label": "Due date",
            "type": "date",
            "default": "2026-10-12",
            "shown_when": {
                "mode": "due"
            }
        },
        {
            "name": "lmp",
            "label": "First day of your last period",
            "type": "date",
            "default": "2026-01-05",
            "shown_when": {
                "mode": "lmp"
            }
        },
        {
            "name": "cycle",
            "label": "Usual cycle length",
            "type": "number",
            "default": "28",
            "unit": "days",
            "shown_when": {
                "mode": "lmp"
            }
        }
    ],
    "outputs": [
        {
            "key": "conception",
            "label": "Estimated conception",
            "format": "text"
        },
        {
            "key": "window",
            "label": "Likely window of intercourse",
            "format": "text"
        },
        {
            "key": "due_date",
            "label": "Due date",
            "format": "text"
        },
        {
            "key": "lmp_equivalent",
            "label": "Equivalent last-period date",
            "format": "text"
        }
    ],
    "formula": {
        "text": "From a due date: conception ≈ due date − 266 days.  From a last period: conception ≈ LMP + (cycle length − 14);  due date = conception + 266 days.  Window = conception − 5 days to + 1 day.",
        "variables": {
            "LMP": "first day of the last menstrual period",
            "266 days": "average time from conception to birth",
            "280 days": "average time from LMP to birth (Naegele’s rule, 28-day cycle)"
        }
    },
    "worked_example": {
        "inputs": {
            "mode": "due",
            "due_date": "2026-10-12",
            "lmp": "2026-01-05",
            "cycle": "28"
        },
        "steps": [
            "Conception: 12 Oct 2026 − 266 days = 19 Jan 2026.",
            "Window: 5 days before (14 Jan) to 1 day after (20 Jan), because sperm can survive several days.",
            "Equivalent LMP: 12 Oct 2026 − 280 days = 5 Jan 2026."
        ],
        "result": "A due date of 12 October 2026 points to conception around 19 January 2026, from intercourse most likely between 14 and 20 January."
    },
    "interpretation": "The estimate is the most likely date, not a certainty. Since sperm can survive up to about a week, intercourse several days before ovulation can lead to pregnancy, which is why a window is shown.",
    "assumptions": [
        "Ovulation (and conception) occurs 14 days before the next expected period.",
        "Pregnancy lasts an average of 266 days from conception (280 days from the last period)."
    ],
    "limitations": [
        "Even with an ultrasound-based due date the conception estimate is uncertain by several days; with an LMP-based date it can be off by a week or more.",
        "Cannot establish paternity: conception dates are too imprecise for that.",
        "IVF pregnancies have known transfer dates; use those instead."
    ],
    "rules": null,
    "faqs": [
        {
            "question": "Can this tell me the exact day I conceived?",
            "answer": "No. Ovulation timing varies and sperm survive several days, so dates are estimates with a window of about a week."
        },
        {
            "question": "Why is my due date 280 days after my period, not 266?",
            "answer": "Pregnancy is conventionally dated from the first day of the last period, about two weeks before conception in a 28-day cycle, so the two counts differ by 14 days."
        }
    ],
    "sources": [
        {
            "title": "StatPearls (NCBI Bookshelf): Estimated Date of Delivery",
            "url": "https://www.ncbi.nlm.nih.gov/books/NBK536986/"
        },
        {
            "title": "Tommy’s: Ovulation and fertility",
            "url": "https://www.tommys.org/pregnancy-information/planning-pregnancy/how-get-pregnant/ovulation-and-fertility"
        }
    ],
    "related": [
        {
            "name": "Ovulation Calculator",
            "url": "https://calculators.geekmidst.com/calculators/ovulation-calculator"
        },
        {
            "name": "Period Calculator",
            "url": "https://calculators.geekmidst.com/calculators/period-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"
}