Conception Date Calculator

Work backward from a due date, or forward from a last-period date, to estimate when conception most likely happened and the few days in which intercourse could have led to it. Conception follows ovulation, which in a regular cycle comes about 14 days before the next period — so the answer is a short window, not an exact day.

How to use this tool

1. Choose whether you know the due date or the first day of your last period.
2. Enter that date.
3. For the last-period method, enter your usual cycle length.
4. Read the estimated conception date, fertile days and timeline.

Formula

Conception ≈ due date − 266 days, or last period + (cycle length − 14) days

A pregnancy lasts about 266 days from conception, or 280 days from the last period in a 28-day cycle. The phase between ovulation and the next period is fairly steady at about 14 days, so longer cycles move ovulation later. Sperm can survive up to five days, which is why the fertile window opens five days before ovulation; the egg lives for about a day. The first day of bleeding is cycle day 1, so adding 18 elapsed days gives cycle day 19. A due-date-derived last-period date is only a dating equivalent.

Example

With a due date of October 8, 2026, conception most likely happened around January 15, 2026, with a fertile window of January 10–15. If instead the last period began on January 1, 2026 and the cycle is 32 days long, ovulation falls on cycle day 19 — January 19, 2026 — and the due date moves to October 12, 2026.