Pregnancy Conception Calculator

Estimate when conception most likely happened — working forward from the last period, or backward from a due date.

Starting point

Estimate from

First day of last period

Average cycle length (days)

Result

Pick a date and press Calculate.

Why a window, not a day

Ovulation typically happens about 14 days before the next period, and sperm can fertilize for several days, so conception is only ever known to within a window of about a week. From a due date, conception is taken as 266 days earlier. These are estimates, not medical findings.