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.

See also