How the estimated due date is calculated, and how accurate it is

The standard rule: 280 days from the last period

Most due dates use Naegele's rule: take the first day of the last menstrual period and add 280 days, which is 40 weeks. Pregnancy is counted from that day even though conception happens about two weeks later.

This is why a woman is already counted as two weeks pregnant on the day conception happens. The count describes the gestational age, not the age of the embryo.

When your cycle is not 28 days

The rule assumes a 28 day cycle with ovulation around day 14. If your cycle is longer or shorter, ovulation usually moves with it, so the date is shifted by the difference. Our calculator has a cycle length field for this reason.

Calculating from the conception date

If you know the conception date, for example from ovulation tracking or fertility treatment, the due date is about 266 days after it. This does not depend on cycle length.

Why it is only an estimate

Few babies arrive exactly on the estimated date. Birth between 37 and 42 weeks is considered normal term, so the date is best read as the middle of a window.

The date can also be off if the last period date is uncertain, if cycles are irregular, or if ovulation happened earlier or later than expected.

The ultrasound can change the date

Your doctor may adjust the due date after an early ultrasound, especially when the scan and the last period date differ noticeably. If your doctor gives you a date, follow it over any calculator.

A worked example

Say your last period started on 1 March 2026 and your cycle is 28 days. Adding 280 days gives 6 December 2026. If your cycle is 35 days, ovulation is about a week later than average, so the date moves a week to 13 December 2026.

Our pregnancy calculator does this for you, and also shows the week you are in, the trimester and a check-up schedule.

If you pass your due date

Going past the due date is common and does not mean something is wrong. Your doctor or midwife will usually see you more often and may talk to you about options such as starting labor. Call right away if the baby moves less than usual, if your waters break or if you have any warning sign.

Making good use of your date

A due date is most useful as a planning tool.

• Tell your doctor the first day of your last period at the first visit
• Write the date down, but plan for a window of a few weeks on either side
• Use the WHO check-up schedule and add the dates to your calendar
• Plan your hospital bag and leave from work with some flexibility