Conception Date Calculator

Related pregnancy tools: Due date · Pregnancy weeks · Trimester dates · IVF due date · Ovulation estimate · Conception estimate

The calculated date is approximate. It is not an exact conception date, a confidence interval or a way to establish paternity.

About this calculator

From an estimated due date, this calculator subtracts 266 days. From a last-period date, it estimates ovulation using cycle length minus 14 days. These are calendar assumptions: they do not identify the actual moment of fertilisation.

A date of intercourse is not necessarily the date of fertilisation. This calendar estimate is not an individual confidence interval. For guidance on pregnancy dating, see ACOG's dating guidance. Date arithmetic cannot establish paternity.

Sources checked 9 October 2026. This tool has not been clinically reviewed.

Frequently asked questions

How is the conception date calculated?

From a due date, the tool subtracts 266 days. From a last period, it adds cycle length minus 14 days to estimate ovulation. Both results are approximate, not observations of fertilisation. There is no calculated individual confidence interval.

Why is conception about two weeks after my last period?

Pregnancy is dated from the first day of the last menstrual period, but conception does not happen until ovulation, which is roughly two weeks later in a typical cycle. So at the moment of conception a pregnancy is already counted as about two weeks along. This is a long-standing medical convention, used because the last period is a date most people can identify, while the exact day of conception usually cannot be known.

How accurate is a conception date calculator?

It cannot give an exact conception date. Its result depends on the due date or cycle assumptions you supply, and those can be uncertain. Follow your healthcare provider's pregnancy dating rather than treating this estimate as proof of a specific date.

Can a conception calculator tell me who the father is?

No. This calculator cannot establish paternity or rule someone in or out. Dates alone cannot resolve parentage.

Is the conception date the same as the ovulation date?

They are very close. Conception, the fertilisation of the egg, happens at or within a day of ovulation, because the egg is viable for only about a day after it is released. So for practical purposes the calculator treats the conception date and the ovulation date as the same when working from a last period. Any difference is a matter of hours rather than days, far smaller than the overall uncertainty in the estimate.

Last updated 2026-10-17