How Due Dates Are Calculated

An estimated due date is one of the first numbers you are given in pregnancy, and it is more approximate than its confident presentation suggests. This guide covers how it is worked out and what the method assumes.

The 280-day rule

The standard method is Naegele's rule, the approach endorsed by the American College of Obstetricians and Gynecologists (ACOG):

Estimated due date = first day of your last menstrual period + 280 days

280 days is 40 weeks. You may also see it expressed as "add nine months and seven days," which gives the same answer.

Two things about that starting point often surprise people. It is the first day of your last period, not the last day. And it is counted from before conception occurred — which means the first couple of weeks of a "pregnancy" measured this way predate the pregnancy itself. That is a convention, not a mistake: the date of the last period is something most people can identify, whereas the date of conception usually is not.

A worked example

If the first day of your last menstrual period was 1 January 2026:

1 January 2026 + 280 days = 8 October 2026

Or by the nine-months-and-seven-days route: nine months from 1 January is 1 October, plus seven days is 8 October. Same result.

Our Pregnancy Due Date Calculator does this for you, and also shows how many weeks along you currently are.

What the method assumes

Naegele's rule assumes a 28-day cycle with ovulation around day 14. That is a reasonable average, and it is an average — real cycles vary considerably between people and from month to month for the same person. If your cycles are consistently longer or shorter than 28 days, the estimate shifts accordingly; what if your cycle isn't 28 days covers that.

It also assumes you know the date of your last period with reasonable confidence. If you are unsure of it, the estimate inherits that uncertainty directly.

Why your clinic's date may differ

If a scan has given you a due date that differs from the one calculated here, the scan date is the more reliable one. Early ultrasound measures the developing pregnancy directly rather than inferring timing from cycle dates, and ACOG treats first-trimester ultrasound as more accurate than LMP-based calculation.

A difference of a few days between the two methods is common and not a cause for concern in itself. Your maternity care team will tell you which date they are using — that is the one your care will be planned around.

Where to go next

How accurate is your due date covers what the estimate can and cannot tell you about when labour will actually start. Pregnancy weeks and trimesters explains how gestational age is counted from the same starting point.

You may also want the Pregnancy Week-by-Week Calculator for where you are now, or the Pregnancy Weight Gain Calculator.

The bottom line

Add 280 days to the first day of your last period. The method is simple, standard, and endorsed by ACOG — and it is an estimate built on an assumed 28-day cycle. If you have had an early scan, trust its date over this calculation, and treat any due date as the middle of a range rather than a fixed day.