Conception Date Calculator
Given a due date, the most likely conception date is 266 days earlier — the average gestational age at birth measured from fertilization.
Estimated dates
Most likely conception
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Conception window
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First day of last period
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Two ways to date a pregnancy
Doctors use gestational age (LMP — last menstrual period — based dating), which starts the clock at the first day of the last period, even though fertilization happens 2 weeks later. A "40-week pregnancy" is gestational. Actual fetal age from fertilization is 38 weeks (266 days).
So if your due date is 40 weeks from LMP, fertilization happened about 2 weeks after the start of the cycle, or 266 days (~38 weeks) before the due date.
Why this is approximate
The 266-day average assumes:
- A 28-day cycle with ovulation on day 14
- Full-term delivery at 40 weeks (only ~5% of births land exactly on the due date)
- Birth at gestational age 40w 0d (the median; ±10 days is normal)
Likely true conception is anywhere in a ±5 day window. If you tracked ovulation directly via OPK or BBT, that date is more accurate than this calculation.
Estimate — not medical advice
Conception dates from this page are approximate and should not be treated as evidence. The fertile window spans several days and implantation timing varies, so a back-calculated date can be off by a week or more.
This is an estimate, not medical advice, and not a diagnosis. It doesn't know your history, your medications, or your other results, and it should never be used to start, stop, or change treatment. If a number here concerns you, take it to a doctor or pharmacist rather than acting on it.