Conception Date Calculator

Conception happens when an egg is fertilised, usually within a day of ovulation. It is almost never observed directly, so it is estimated from dates that are known: a due date, or the first day of a last period.

This calculator gives both a single most likely date and the window around it, because intercourse several days before ovulation can still be what led to the pregnancy.

These calculators provide estimates for informational purposes only. They are not intended to diagnose, treat, prevent, or replace professional medical advice. Pregnancy and menstrual cycles can vary, so results may not be exact.

How this calculator works

What you need to enter:

Your estimated due date, or the first day of your last period with your cycle length

Working back from a due date

A due date marks 40 weeks of gestational age, which is counted from the last menstrual period. Conception happens about two weeks into that count, so a pregnancy runs roughly 266 days from conception rather than 280.

Subtracting 266 days from the due date therefore gives the most likely conception date. If your due date came from an early ultrasound rather than your dates, this estimate inherits that scan's accuracy, which is usually good.

Why there is a window, not a date

Sperm can survive up to about five days in the reproductive tract, so intercourse days before ovulation can still result in conception. Working backwards, the act that led to a pregnancy could have happened anywhere in the five days before the estimated conception date, or on the day itself.

The calculator shows that window alongside the single date. The single date is the best point estimate; the window is the honest range.

When people ask this question

Two situations come up most often. One is curiosity about the timeline of a confirmed pregnancy. The other is wanting to relate a pregnancy to a particular date.

For the second, it is worth being clear about what a calculator can and cannot do. This is an estimate derived from average intervals. It is not a determination of paternity, and it should not be used as one. Only medical or laboratory testing can answer that question.

A worked example

An estimated due date of 2 April 2027.

Subtract 266 days from 2 April 2027, giving 10 July 2026.
That is the most likely conception date.
Add the five-day sperm survival span before it and one day after.
The conception window becomes 5 to 11 July 2026.

The equivalent last menstrual period would have started around 26 June 2026, 280 days before the due date.

What this calculator can't tell you

Conception timing cannot be known precisely from dates. Both the single date and the window are estimates.
The estimate depends entirely on the accuracy of the due date or last period you enter.
Ovulation does not always happen at a typical point in the cycle, which shifts conception with it.
This calculator must not be used to establish paternity. Only medical or laboratory testing can do that.

What to do next

Confirm the due date this works back from with the due date calculator.
See where the conception date sits on the full timeline in the pregnancy calculator.

Common questions

How is the conception date calculated from a due date?

By subtracting 266 days. A due date is 280 days from the first day of the last period, and conception happens about 14 days into that span, leaving roughly 266 days from conception to the due date.

How accurate is a conception date estimate?

It is a reasonable estimate rather than a precise one. It assumes conception occurred close to ovulation and that ovulation happened at a typical point in the cycle. Where a due date came from an early ultrasound, the estimate is more dependable than one based on a remembered last period.

Why is the conception window six days wide?

Because sperm can survive for up to about five days, intercourse in the days leading up to ovulation can still cause conception. The window covers those days plus the estimated conception date itself.

Can this calculator determine paternity?

No. It produces an estimated range from average intervals and cannot establish who a biological parent is. Paternity can only be determined by appropriate medical or laboratory testing.

Is the conception date the same as the implantation date?

No. Conception is fertilisation, at or near ovulation. Implantation happens later, typically 6 to 12 days afterwards, when the embryo attaches to the uterine lining. The implantation calculator covers that separately.