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Conception usually happens within a day of ovulation, which is about 14 days before the next period would have started. Working back from a due date gives a conception date about 266 days (38 weeks) earlier.
Because sperm can live up to 5 days, intercourse up to 5 days before ovulation can lead to pregnancy. The calculator shows the most likely conception date and the full window.
| Calculate From | Due date |
| Date | 25 Dec 2026 |
| Cycle Length | 28 days |
| Most likely conception date | Fri, 3 Apr 2026 |
| Possible conception window | Sun, 29 Mar 2026 – Sat, 4 Apr 2026 |
| Fertile intercourse most likely between | Sun, 29 Mar 2026 and Fri, 3 Apr 2026 (sperm live up to 5 days) |
| Calculated from | due date Fri, 25 Dec 2026 |
| Matching LMP date | Fri, 20 Mar 2026 |
Conceived about Fri, 3 Apr 2026.
Try this example →| Calculate From | Last menstrual period |
| Date | 1 May 2026 |
| Cycle Length | 30 days |
| Most likely conception date | Sun, 17 May 2026 |
| Possible conception window | Tue, 12 May 2026 – Mon, 18 May 2026 |
| Fertile intercourse most likely between | Tue, 12 May 2026 and Sun, 17 May 2026 (sperm live up to 5 days) |
| Calculated from | last period Fri, 1 May 2026 and a 30-day cycle |
| Due date | Sun, 7 Feb 2027 |
Sun, 17 May 2026.
Try this example →Usually within a few days to a week. Dates from an early ultrasound are more reliable than period dates.
38 weeks (266 days).
Not exactly — ovulation timing varies and sperm survive several days, so think in terms of a 6-day window.