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Works backwards from a 40-week pregnancy: conception typically occurs about 266 days (38 weeks) before the due date, roughly two weeks after the last period began. Assumes a 28-day cycle; longer or shorter cycles shift the window. An estimate, not a medical determination.
What The Conception Date Calculator Works Out
- Likely conception date: pregnancy dating runs on a 40-week clock that starts at the first day of the last period, about two weeks before conception itself. Reversing it, conception typically falls 266 days (38 weeks) before the due date.
- Probable window: five days either side of that date by default. Sperm survive up to five days and the egg about one, so the honest answer is a window of several days around ovulation rather than a single date.
- Last period likely began: the due date minus 280 days on a 28-day cycle, the start of the pregnancy clock the NHS uses.
- Birth date instead of due date (advanced): only about 4% to 5% of babies arrive on their due date. Enter the actual birth date with the weeks and days of pregnancy at birth (from the red book or discharge notes) and the calculator rebuilds the due date first, which removes that scatter.
- Cycle and luteal phase length (advanced): the standard sums assume a 28-day cycle with ovulation around day 14. Ovulation in a 35-day cycle comes about a week later, so the last period began earlier than the 280-day rule suggests; the conception date itself is unchanged because it is tied to the due date.
- Window size (advanced): widen or narrow the probable window. Due dates are themselves estimates that the 12-week scan often adjusts by a few days.
- Intercourse most likely between (advanced): the five days up to and including the conception date, since sperm can wait for the egg but the egg cannot wait for sperm.
- Pregnancy test positive from (advanced): the first day of the missed period, about 14 days after conception; early-result tests can show from roughly 10 days.
- Cycle day and implied due date (advanced): the day of the cycle on which conception most likely happened, and the due date the calculation rests on. For anything with legal weight, such as paternity, date arithmetic gives a window, not proof, and DNA testing is the reliable route. For dating a current pregnancy, the NHS uses the last period date and the dating scan.
Source and further reading: NHS: working out your due date.