How Due Dates Are Calculated — and What They Actually Predict

The 280-day model

The conventional estimated due date is the first day of the last menstrual period plus 280 days — forty weeks. Some versions adjust for cycle length, adding or subtracting the difference from 28 days, on the reasoning that a longer cycle means later ovulation.

The model embeds two assumptions: a 28-day cycle and ovulation on day 14. Both are averages rather than facts about any individual pregnancy, which is why the resulting date is an estimate produced by a convention, not a measurement.

Use this guide with the right tool: Open the Pregnancy Due Date Calculator for an estimated due date from the last menstrual period and cycle length. If the question shifts, compare it with the Conception Date Calculator or the Pregnancy Week Calculator.

What it does and does not predict

The due date marks the centre of a distribution. Births between 37 and 42 weeks are all within the normal range, and only a small minority occur on the estimated date itself. Presented as a single date, it invites a precision the underlying biology does not have.

This matters for planning rather than for care. Arrangements built to start or end exactly on the due date have no tolerance in either direction, and the direction that causes problems — arriving early — is the one people plan for least.

When the date changes

A dating scan, usually in the first trimester, measures the embryo directly. Where that measurement disagrees with the LMP-based date by more than a defined tolerance, the scan date generally supersedes it, because a direct measurement beats an inference from a cycle assumption.

When the due date is revised, every derived figure moves with it: gestational age, days remaining, the trimester position and any milestone dates already noted. Recalculate the whole set from the new date rather than adjusting individual numbers, or the record ends up internally inconsistent with no signal which entry is stale.

Using the calculator honestly

The calculator applies the standard model exactly and shows the assumptions it used, which makes it useful for understanding how a date was derived, checking that two figures you have been given are consistent, or seeing the effect of a cycle-length adjustment.

It does not date a pregnancy. That is done by the midwife or obstetrician managing it, using clinical assessment and ultrasound. Where the calculator and the notes disagree, the notes are right and the difference is worth asking about rather than resolving yourself.

Worked example

The shape of the calculation stays the same with your own figures. What changes is the source of the rule, so check that before treating the number as settled.

Common mistakes to avoid

• Entering the last day of the period. The model counts from the first day of the last menstrual period. Using the last day shifts the date by several days.
• Treating the due date as a deadline. It is the centre of a range spanning several weeks. Build plans with tolerance in both directions.
• Keeping the LMP date after a scan revision. When dating is revised, recalculate everything from the new date rather than adjusting figures individually.

Use the right calculator

Start with whichever matches the information you already hold. The other two are there for when the question shifts.

Pregnancy & PediatricPregnancy Due Date Calculatoran estimated due date from the last menstrual period and cycle length

Pregnancy & PediatricConception Date Calculatoran estimated conception window from LMP or due date

Pregnancy & PediatricPregnancy Week Calculatorpregnancy week and day on a selected date

Frequently asked questions