How the calculator works
A typical pregnancy lasts about 40 weeks counted from the first day of the last period, but conception happens about two weeks into that count. So a baby born on their due date was conceived about 266 days (38 weeks) before birth. The calculator counts back from the date of birth to estimate the conception date.
Because only a small share of babies arrive exactly on their due date, the calculator also shows a likely range. Without extra information, it covers births anywhere from 37 to 42 weeks. If you know how many weeks pregnant the mother was at birth — from a baby book, birth record or family story — enter it for a narrower, more accurate estimate.
Birth month to likely conception month
| Born in | Likely conceived in |
|---|---|
| January | April–May (previous year) |
| March | June–July (previous year) |
| May | August–September (previous year) |
| July | October–November (previous year) |
| September | December–January |
| November | February–March |
For a full-term birth, the conception month is usually about nine months earlier, give or take a few weeks.
Why it can only be an estimate
Several things make the exact day impossible to know: babies are born across a five-week window around the due date; ovulation does not always happen on day 14 of the cycle; and sperm can survive for several days, so conception can happen days after intercourse. Babies born early were conceived closer to their birth date than the standard estimate, and babies born late, further from it.
If you are pregnant now and want to know when you conceived, the Conception Date Calculator works from your last period or due date instead.