Pregnancy Due Date Calculator — Estimated Delivery Date

Estimate a due date from a last period, conception date or an early scan, with milestones.

A due date is estimated as 280 days from the first day of the last menstrual period, or 266 days from conception — a rule of thumb from the 1800s known as Naegele's rule. It assumes a 28-day cycle with ovulation on day 14, so a longer or shorter cycle shifts the date. Fewer than 5% of babies are born on the estimated date.

Runs in your browser

Basis: 280 days from the last menstrual period, or 266 from conception
Accuracy: About 90% of births fall within two weeks either side
Privacy: Runs entirely in your browser — nothing is uploaded
Cost: Free, unlimited, no sign-up

Frequently asked questions

Why is pregnancy dated from my last period rather than conception?

Because the date of the last period is usually known and the date of conception rarely is. It means the first two weeks of a "40-week pregnancy" happen before conception, which is why gestational age always runs about two weeks ahead of the embryo's actual age.

My cycle is 35 days — does that change my due date?

Yes, by about a week. Naegele's rule assumes ovulation on day 14; with a 35-day cycle it is nearer day 21, so conception and the due date both shift later. Setting your real cycle length above applies that adjustment.

Should I trust this over my ultrasound date?

No. An early ultrasound, particularly before 14 weeks, measures the embryo directly and is more accurate than any date-based rule — which is why clinicians revise the due date when the two disagree by more than about a week. This calculator is an estimate, not medical advice.

How to use the pregnancy due date calculator

1Choose the calculate from.
2Enter or paste your date.
3Set the average cycle length.
4The result appears immediately — copy or download it.

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Last updated 1 Aug 2026

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