How the Conception Date Calculator works
The calculator works backwards from your pregnancy dates. From your due date it subtracts 266 days (38 weeks). From the first day of your last period it adds about 14 days — adjusted for your cycle length, because ovulation normally happens about 14 days before the next period is due. It also shows the likely fertile window: the five days before ovulation and the day after, when intercourse is most likely to have led to conception.
Why conception is usually an estimate
Several things make the exact date hard to pin down. Sperm can survive in the reproductive tract for up to about five days, so conception can happen days after intercourse. The egg survives for roughly 12–24 hours after ovulation. And ovulation itself does not always happen on the same cycle day, even for people with regular cycles. That is why pregnancy is dated from the last period rather than from conception, and why this calculator presents a likely window as well as a single date.
Conception date from a due date: an example
If your estimated due date is September 1, 2027, subtracting 266 days gives an estimated conception date of December 9, 2026, with a likely fertile window from December 4 to December 10, 2026. If your due date was set by an early ultrasound, this estimate is usually within a few days of the true date.
Gestational age vs conception age
Because pregnancy is counted from the last period, the baby is about two weeks younger than the pregnancy. At 12 weeks pregnant, for example, the baby is around 10 weeks from conception. Ultrasound reports and pregnancy week pages use gestational age. If you know your conception date and want your due date, use the Due Date Calculator by Conception.