Conception Date Calculator

In a cycle with a textbook 28-day length, ovulation — and with it, conception — happens about 14 days after the first day of the last menstrual period (LMP). That is the default this calculator uses: conception ≈ LMP + 14 days. It is the same 14-day assumption behind the standard 280-day due-date estimate, just pointed at a different question.

A due date is 280 days after the last period, and conception is about 266 days before the due date. So if you know your due date (say from an ultrasound), conception ≈ due date − 266 days. Switch the method above to use this — it can be more accurate than the last-period estimate when the due date came from a scan.

Both methods assume a fairly regular cycle, with ovulation landing on day 14. Real cycles do not always cooperate: the timing of ovulation can shift by several days from one cycle to the next, even in someone whose cycles are usually consistent, because the first half of the cycle — before ovulation — is the part that actually varies. The further a real cycle sits from that 28-day assumption, the less precise the single "most likely" date becomes, which is exactly why the result is shown as a window rather than a fixed point.

Conception cannot be pinned to a single certain date from a calendar alone. Sperm can remain viable inside the reproductive tract for three to five days, and the egg itself is fertilizable for only about 24 hours after ovulation, so fertilization could happen on any of several days around the estimated date. The window this calculator shows — three days before the most likely date to one day after — reflects that biology. Only a medical dating method, like an early ultrasound, narrows it further.

A first-trimester ultrasound measures the embryo directly — typically its length, crown to rump — and dates a pregnancy more precisely than any calculation from the last period or a due date, because it is based on measured growth rather than an assumed cycle. If a clinician's dating scan gives a different date than this calculator, the scan is the more reliable number. Treat this estimate as a personal reference point, useful for your own records or for questions to bring to a prenatal visit — not as a medical or legal determination of conception, and not a substitute for prenatal care.