How is the due date worked out?

Last period plus 280 days (40 weeks). From conception, add 266 days.

It is an estimate. Your midwife or doctor may adjust it after an ultrasound scan.

What if my cycle isn't 28 days?

The estimate assumes a 28-day cycle. Tell your care provider if yours differs.

All tools

Pregnancy Due Date Calculator

Enter the first day of your last period, or your conception date, to estimate your due date.

Calculation Method

First Day of Last Period

Estimate your pregnancy due date from your last menstrual period (LMP) or conception date — for expectant parents.

Formula

Due date = LMP + 280 days (40 weeks). From conception: add 266 days (38 weeks).

Reference Values

•Full-term pregnancy = 40 weeks from LMP
•Conception occurs ~14 days after LMP
•Due dates have ±2 week accuracy

FAQ

Common questions about Pregnancy Due Date Calculator

How we calculate this

We use the standard published formula for this calculation, and we show the working so you can check it.

Spotted a mistake? Tell us and we'll check it within a few days. See how we check our calculators.

Medical disclaimer

This tool gives a general estimate for information only. It is not medical advice and cannot diagnose a condition. Talk to a doctor, midwife or other qualified professional about your own situation.