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MasterMath

Conception Date Calculator

With a due date of 25 December, conception was around 3 April, within a seven-day window. Work out when it happened from the due date or from your last period.

Conception could have been between

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Conception could have been between—
Likeliest day—
Last period—
Estimated due date—
Weeks pregnant today—
Real age of the embryo—

How this was worked out

    Indicative result. This is an estimate based on a regular cycle and ovulation on day 14. Only an early scan can narrow down the real date of conception, and in questions of paternity no calculator counts as evidence.

    The formula

    conception ≈ last period + 14 days · last period = due date − 280 days

    Where it comes from

    Pregnancy is counted from the first day of the last period, not from conception, because that is the date remembered with certainty. Ovulation comes about two weeks later, and that is when fertilisation can happen. That is why gestational age always runs two weeks ahead of the embryo's real age.

    How to work it out by hand

    1. If you start from the due date, subtract 280 days to reach the last period
    2. Add 14 days to the last period to place ovulation
    3. That is the estimated date of conception
    4. Extend five days back and one forward for the real window

    What is worth knowing

    Giving a single day is misleading. Sperm survive up to five days inside the body, so intercourse nearly a week earlier may be what fertilised the egg, which itself lives only about twenty-four hours. The honest window is those seven days. The calculation also assumes you ovulate on day 14, and that only holds on 28-day cycles: on longer cycles ovulation is later and everything shifts, which is why the cycle is a field on the form. If your cycle is irregular, the only way to narrow it down is a first-trimester scan, which estimates gestational age by measuring the embryo.

    Frequently asked questions

    How do I know when I conceived?

    Subtract 266 days from the estimated due date, or add 14 to your last period. The result is an estimate with a window of about seven days.

    Why is pregnancy counted from the last period?

    Because it is the date remembered for certain. Conception happens about two weeks later, so at week 8 of pregnancy the embryo is 6 weeks old.

    Can it establish who the father is?

    No. The window covers several days and the margin of error runs to weeks on an irregular cycle. Only a DNA test does that.

    What if my cycle is not 28 days?

    Enter its real length: ovulation moves with it. On a 35-day cycle you ovulate around day 21, not 14, and conception is a week later.

    Which date is more reliable, mine or the scan's?

    The first-trimester scan, which measures the embryo directly. If it differs much from your count, the due date is corrected to the scan's.