Conception Date Calculator — when did I conceive?

How it works

A due date is set 40 weeks — 280 days — after the first day of the last menstrual period (LMP). Conception, however, occurs later, at ovulation, which the Office on Women's Health places about two weeks after the LMP. Subtracting those two weeks gives a conception estimate of roughly the due date minus 266 days (280 − 14).

Conception is not a single guaranteed instant. Sperm "can live for up to three days" in the body, and the egg is fertile for about a day after ovulation, so the actual conception could fall across a short window — from about three days before the estimated date to one day after.

The calculator also shows the implied last period (due date − 280 days), which is useful if you know your due date from an ultrasound but not your LMP.

These are estimates. Ovulation timing varies with cycle length, and an early ultrasound dates a pregnancy more precisely than either the LMP or a back-calculated conception date.

Worked example

For a due date of October 8, 2026, subtract 266 days to estimate conception around Jan 15, 2026. The fertile window runs from about Jan 12, 2026 to Jan 16, 2026, and the implied last period is Jan 1, 2026 (280 days before the due date).

Frequently asked questions

How do you calculate conception date from a due date?

Subtract 266 days from the due date. A due date is 280 days after the last period and conception happens about 14 days after the last period, so 280 − 14 = 266 days before the due date. For an October 8, 2026 due date, that is about January 15, 2026.

Is the conception date exact?

No. It marks the most likely day of ovulation, but conception can occur across a window of a few days because sperm survive up to three days and the egg stays fertile for about a day. The calculator shows this window.

Why is conception about two weeks after my last period?

Pregnancy is dated from the first day of the last period, but you do not ovulate until around day 14 of an average cycle. Conception happens at ovulation, so it lands roughly two weeks into the 40-week count.

Can I find conception from my last period instead?

Yes. If you know the first day of your last period, conception is about 14 days later. The implied-last-period output also works the other way, estimating your LMP from the due date.

How does cycle length affect the estimate?

The 266-day rule assumes ovulation around day 14. If your cycles are longer or shorter, ovulation — and therefore conception — shifts accordingly, moving the estimate by a few days.

Why might this differ from my ultrasound date?

An ultrasound measures the baby's size directly and is generally the most accurate way to date a pregnancy, especially in the first trimester. If it disagrees with the LMP-based estimate, providers usually go by the ultrasound.