When did conception most likely happen?

How is conception date estimated?

From a known due date, conception is estimated as due date minus 266 days. From a known last menstrual period (LMP), conception is estimated as LMP plus 14 days, since ovulation and conception typically occur around two weeks into a standard 28-day cycle. Both routes are offered here since people commonly know one date but not the other.

Why is it different from the due date minus 40 weeks?

Due date minus 40 weeks (280 days) gets you back to the first day of the last menstrual period, not conception itself — that's the calculation the Pregnancy Due Date Calculator uses in reverse. Conception happens roughly two weeks after LMP, so the conception estimate here uses 266 days instead of 280 to land on the more likely actual date of conception rather than the start of that cycle.

How accurate is this?

It's an estimate based on average cycle timing, not a precise measurement — actual ovulation and conception timing varies by individual cycle length, and can shift the real date by several days in either direction, especially for people with irregular cycles. Treat the result as a reasonable window to think about, not an exact date. See the Ovulation Calculator if you're trying to estimate a fertile window going forward rather than looking back.

How does this relate to figuring out paternity timing questions?

This gives a general estimate useful for personal timeline and planning purposes, but it is not precise enough for legal or paternity determinations, which rely on DNA testing rather than date-based estimation. Use this tool for general context only, not as evidence for any formal timing question.

Worked example: a due date of January 15, 2027 implies an estimated conception date of January 15, 2027 minus 266 days ≈ April 24, 2026. A last period date of June 1, 2026 implies an estimated conception date of June 1, 2026 plus 14 days = June 15, 2026.