When Did I Conceive?

Calculate based on

First day of your last period

Average cycle length (days)

Estimated (or actual) due date

Most likely conception date

possible window

estimated due date

Conception is estimated from ovulation, which usually happens about 14 days before the next period — timing varies from cycle to cycle, so treat this as a reasonable estimate rather than an exact date. An early ultrasound gives the most accurate dating.

How this is calculated

Ovulation is typically followed by a fairly consistent "luteal phase" — the roughly 14-day stretch between ovulation and the next period. It's the phase before ovulation (the follicular phase) that actually varies from person to person and cycle to cycle. That's why the last-period method here works backward from your next expected period (last period date plus cycle length) and subtracts 14 days, rather than just adding a fixed number of days to your last period.

The due-date method reverses the standard pregnancy-dating convention: since due dates are usually estimated as 266 days after conception, conception is estimated as your due date minus 266 days.

The possible window shown alongside your most likely conception date accounts for the fact that sperm can survive in the reproductive tract for several days while waiting for ovulation, while the egg itself is generally only viable for about 24 hours after release.

Common edge cases

The last-period method depends heavily on knowing your average cycle length accurately. If your cycles are irregular — common with PCOS, thyroid conditions, high stress, or in the months after stopping hormonal birth control — the 14-days-before-next-period assumption becomes much less reliable, since the "next period" itself is harder to predict.

Breastfeeding can suppress or delay ovulation unpredictably, sometimes for many months, which throws off cycle-based estimates entirely. Perimenopause can have a similar effect as cycles become less predictable.

If you were actively tracking ovulation at the time — with basal body temperature, ovulation predictor kits, or cervical mucus changes — that observed date will generally be more accurate than any date calculated from your last period alone.

Sources

The 266-day conception-to-due-date convention and luteal-phase assumptions used here reflect standard obstetric practice. For more on ovulation, fertility, and pregnancy dating, see the American College of Obstetricians and Gynecologists (ACOG), Mayo Clinic, and NICHD.