Conception Date Calculator

Conception happens at ovulation, about 14 days after the first day of your last period and roughly 266 days (38 weeks) before your due date. So from a due date the calculator subtracts 266 days; from your last period it adds about 14 days; from a birth date it subtracts about 266 days. Because exact ovulation timing varies, the result is shown as a window of a few days rather than a single date.

It gives a close estimate, not an exact day. Conception can only happen during the fertile window, and ovulation may occur a few days earlier or later than day 14, especially with irregular cycles. An early dating ultrasound is the most reliable way to estimate conception, usually accurate to within about 5 days.

No. It estimates the most likely conception date and a surrounding window. Sperm can survive up to 5 days, so intercourse on several different days could have led to the same conception. The calculator cannot pinpoint a single act of intercourse or determine paternity.

Conception (fertilisation) is when sperm meets egg, around ovulation. Implantation is when the fertilised egg attaches to the uterine lining, typically 6–12 days after conception. A pregnancy test only turns positive after implantation, when the hormone hCG begins to rise.

In an average 28-day cycle, ovulation — and therefore conception — occurs around day 14, roughly two weeks after the first day of your period. This is why pregnancy dated from the last period (40 weeks) is about 2 weeks longer than the actual age of the baby (38 weeks).

Yes, approximately. Switch to the birth-date mode and the calculator works back about 266 days (38 weeks) from the birth date. If your baby was born early or late, the actual conception date shifts accordingly, so treat the result as an estimate.