Conception date

How it works

Conception happens on average about 266 days (38 weeks) before the estimated due date. Since the exact moment of ovulation varies, the tool also shows a range of a few days.

Note

It's a statistical estimate, not a certainty: real pregnancy length varies. For precise information see your gynecologist.

Why 266 days and not 280

The 280 days everyone talks about (forty weeks) are counted from the first day of the last period, not from conception: that is Naegele's rule, born when the only date a woman could recall with certainty was the one of her cycle. But about two weeks pass between the start of the cycle and ovulation, and conception can only happen after ovulation.

Hence the number this page uses: 266 days, that is 280 minus 14. It is why at the first scan a woman is told she is «six weeks along» when she knows perfectly well the encounter was four weeks earlier: those first two weeks are counted anyway, by convention.

Why it is a range and not a day

Ovulation does not always fall mid-cycle. On long cycles it comes later, on short cycles earlier, and in the same woman it can shift by days from one month to the next because of stress, travel or illness. On top of that sperm survive up to five days in the genital tract: the intercourse that led to conception may have happened well before fertilisation itself.

That is why the result is a window and not a date. A single date would be easier to read and easier to disprove.

When the due date has already been corrected

If the due date you are using comes from a first-trimester scan, the estimate here is firmer: between the seventh and thirteenth week the embryo grows very uniformly and the crown-rump length dates the pregnancy to within a few days, so much so that it overrides the last-period date when the two disagree.

If instead the due date comes only from the menstrual calendar, all the uncertainty of the cycle lands on the result. And in assisted-reproduction pregnancies the calculation is not needed at all: there the transfer date is known to the day.

A question that is often a different question

People looking for a conception date very often want to know something else: who the father is, or whether a particular night «could have been the one». This page cannot answer that, and no calendar can. The window spans several days precisely because biology does not allow it to be narrowed, and the only test that answers a paternity question is a DNA test.

Nearby tools

For counting forwards there is Pregnancy weeks, which tells you which week you are in and when the trimesters fall; for the fertile window Ovulation calculator. After the birth you need Baby age and Growth percentiles.