Conception date and due date: what you can and can't know

Counting a due date from conception is appealingly direct: if you know the day fertilisation happened, you can count forward to the birth. The method is simple. The catch is that, outside of fertility treatment, the exact day of conception is rarely known.

The 266-day rule

A pregnancy runs about 266 days from conception, compared with 280 days from the last menstrual period. The 14-day difference is the typical gap between the start of a period and ovulation on a 28-day cycle. So:

due date = conception date + 266 days

For the gestational-age display, the calculator treats the equivalent last period as 14 days before conception. Enter an ovulation or conception date in conception mode and it does both steps.

How well can you know the conception date?

Fertilisation happens within roughly a day of ovulation, but sperm can survive several days in the reproductive tract, so intercourse and conception aren't the same date. Even people tracking ovulation closely — with cycle apps, temperature, or ovulation tests — usually have a window of a day or two rather than a single certain day. That's why menstrual and ultrasound dating remain the clinical standards for most pregnancies.

The clear exception is IVF, where fertilisation timing is recorded exactly. If that's your situation, use IVF due date calculation instead, which is more precise than a conception estimate.

Conception date vs. “date of the deed”

A frequent misunderstanding is treating the day of intercourse as the conception date. Because sperm can live for up to about five days, conception can occur several days after intercourse. If you're working backwards from a single encounter, the true conception date could be anywhere in that window, which adds uncertainty to the estimate.

When conception dating is useful

It's most helpful when the last period is unknown or unreliable but ovulation was tracked, or as a sanity check against menstrual dating. If the two methods land within a few days of each other, that's reassuring. If they diverge, an early ultrasound settles it — see how accurate is your due date.

The bottom line

Conception dating is a legitimate, simple method — +266 days — but only as good as the conception date you feed it. Treat the result as an estimate to confirm with a clinician, and lean on an early scan if the date matters and the conception day isn't certain.