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Due date calculator

Find your estimated due date and how many weeks pregnant you are today. Choose the date you know best: your last period, the day you conceived, an IVF transfer or an ultrasound scan.

Important: This calculator gives an estimate, not a diagnosis or a confirmation of pregnancy. Cycle variation and the date of ovulation can affect the result, and only about 1 in 20 babies arrives on the estimated due date. An ultrasound or your clinician may give a different due date, and theirs takes priority. Contact your maternity care team for personalised guidance.

Your dates are processed in this page and are not sent to our server or saved by this tool.

How a due date is calculated

A full-term pregnancy lasts about 40 weeks, counted from the first day of your last menstrual period. That is about 38 weeks from conception, because conception happens around two weeks after your period begins. Each method here works back to the same 40-week timeline.

Last period (LMP): we add 280 days to the first day of your last period. If your cycle is longer or shorter than 28 days, the date moves later or earlier to match.
Conception date: we add 266 days, which is 38 weeks.
IVF transfer: we add 266 days and take off the age of the embryo. A day 5 blastocyst transfer therefore adds 261 days, and a day 3 transfer adds 263 days.
Ultrasound scan: we count forward from the gestational age on your scan report to week 40.

Which method is the most accurate?

IVF dates are the most precise because the exact day of conception is known. An early ultrasound, usually offered before about 14 weeks, is also reliable. If a scan and your last period disagree by more than about a week, your midwife or doctor will usually go with the scan.

A date from your last period works well when you remember it and your cycles are regular. A conception date is harder to be sure of unless you tracked ovulation. If you are unsure, use the method that matches the date you are most certain of. Whatever this page shows, the due date your care team gives you takes priority.

Weeks and months of pregnancy

Pregnancy is counted in weeks, so people are often unsure how that fits into months. As a rough guide:

Month 1: weeks 1 to 4
Month 2: weeks 5 to 8
Month 3: weeks 9 to 13
Month 4: weeks 14 to 17
Month 5: weeks 18 to 22
Month 6: weeks 23 to 27
Month 7: weeks 28 to 31
Month 8: weeks 32 to 35
Month 9: weeks 36 to 40

See what is happening each week on our pregnancy week-by-week guide.

Your due date is an estimate

Only about 1 in 20 babies is born on the estimated due date. Most arrive between 37 and 42 weeks, which is why the result above shows a range as well as a date. If you go past your due date, your care team will talk to you about monitoring and whether to offer induction. Our guide to the induction of labour explains what that involves.

What to do next

Read our guide to antenatal appointments and tests to see which scans and checks to expect, and when.
Not sure whether you are pregnant yet? Use the pregnancy test calculator to find the best day to test, or read about early signs of pregnancy.
Planning time off work? See our maternity leave guides for the UK, Ireland, Canada, Australia and the USA.
Trying to conceive? Try the ovulation calculator to estimate your fertile window.

Explore our pregnancy guides or read the NHS pregnancy guidance.