How we estimate the day you conceived
An egg is usually fertilised within a day of being released, and in a standard 28-day cycle ovulation falls about 2 weeks after your last period began. Since pregnancy is dated from that period, conception typically sits around 2 weeks after your LMP — or, counting back from the other end, 266 days (38 weeks) ahead of the due date.
Sperm can stay alive for up to 5 days, so the intercourse that resulted in pregnancy may have taken place as much as 5 days before conception itself. For that reason the result is shown as a window of days, not one date.
Working back from your baby’s birthday
When you enter a birth date, the calculator subtracts 266 days, assuming your baby arrived at full term. Full-term births happen anywhere from 37 to 42 weeks, so the possible conception window spans about 5 weeks. If your baby came early, conception will have been nearer to the birth date.
| Date you have |
Estimated conception |
| Start of last period (28-day cycle) |
LMP + 14 days |
| Due date |
Due date − 266 days |
| Birth date (full term) |
Birth date − 266 days (range −280 to −245) |
No calculator can pin down the exact day of conception or settle questions of paternity; that needs a DNA test.
Common questions
How close is the estimate likely to be?
With regular cycles and a due date taken from an early scan, it is usually right to within a few days. Irregular cycles or an uncertain period date widen the range.
Why can’t it tell me one exact day?
Ovulation does not always happen on the same cycle day, and sperm survive for up to 5 days, so pregnancy could have followed sex up to 5 days before ovulation.
How far along am I counted on the day of conception?
Roughly 2 weeks, since pregnancy dating begins on day one of your previous period.
Does an early birth change the result?
Yes. The birth-date method assumes full term, so if your baby was premature, conception was closer to the birth date than the result suggests.
References: ACOG: Methods for estimating the due date · Timing of intercourse and conception (NEJM 1995)
Checked against the sources listed above. This is general health information and not medical advice — please ask your doctor or a registered dietitian what suits you.
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