Conception Date Calculator 💫
Reverse-calculate your likely conception date from your due date. Find out when your baby was conceived.
How the Conception Date Calculator Works
Our conception date calculator works by reverse-engineering the standard pregnancy timeline. While pregnancy is conventionally measured as 40 weeks (280 days) from the first day of your last menstrual period (LMP), the actual time from conception to birth is approximately 38 weeks (266 days). The two-week difference accounts for the time between the start of your period and the day you actually ovulate and conceive.
By subtracting 266 days from your estimated due date, we arrive at the approximate date when the sperm fertilized the egg — your conception date. This calculation is based on the same obstetric principles used by healthcare providers worldwide. It works in reverse: instead of projecting forward from a known date to find the due date, we project backward from the due date to find the conception date.
The calculator also displays your probable fertile window, which spans from approximately 5 days before the estimated conception date to 1 day after. This is the window during which intercourse most likely led to pregnancy. Sperm can survive in the female reproductive tract for up to 5 days, meaning that intercourse several days before ovulation could still result in conception.
Keep in mind that the conception date is an estimate. Several factors introduce a margin of uncertainty: the accuracy of your due date itself, the possibility that sperm survived multiple days before fertilizing the egg, variations in the luteal phase length, and the difference between a due date calculated from LMP versus one confirmed by ultrasound. For the most reliable results, use a due date that was confirmed by a first-trimester ultrasound.
Understanding your conception date can be useful for several reasons. It satisfies natural curiosity about your pregnancy timeline, can help with understanding genetic screening dates, and provides a more precise foundation for tracking embryonic and fetal development milestones. Some parents enjoy knowing the approximate date their baby was conceived as part of their pregnancy journey and memory-keeping.