Pregnancy Due Date Calculator for Clinicians

Work out gestation and the estimated due date (EDD) from the last period or a dating scan, with key antenatal dates.

By Dr Amer Sheikh, GP Updated 17 October 2026 Clinical review pending

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How the due date is worked out

Naegele’s rule adds 280 days (40 weeks) to the first day of the last menstrual period. A dating scan is more accurate and takes priority.

Key antenatal dates

Milestone	Timing
Booking appointment	Ideally by 10+0 weeks
Dating scan	11+2 to 14+1
Anomaly scan	18+0 to 20+6
28-week review	FBC, antibodies; anti-D if RhD negative
Term	37+0
Due date	40+0
Discuss induction	41+0 (NICE NG207)

Frequently asked questions

How is a due date calculated?

Add 280 days to the first day of the last period, or use dating scan measurements.

Is the scan date or LMP date more accurate?

The dating scan is more accurate and is used if they differ.

When is induction discussed?

NICE NG207 suggests discussing and offering induction from 41+0 weeks.

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References

NHS Fetal Anomaly Screening Programme. gov.uk
NICE. Inducing labour. NG207. nice.org.uk/guidance/ng207

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