Pregnancy tools
Pregnancy due date calculator
Work out your estimated due date from your last period, conception date or scan. Then see your UK antenatal appointments, the dates your employer needs and the 28-week side-sleeping milestone on one timeline.
Your due date appears hereChoose how to work it out and enter a date.
Comfort for the weeks ahead
How your due date is worked out
Pregnancy is counted from the first day of your last period, not from conception. A typical pregnancy lasts 280 days, or 40 weeks, from that day, so the calculator adds 280 days. If your cycle is longer or shorter than 28 days, it moves the date by the difference: a 32-day cycle adds 4 days, and a 25-day cycle takes 3 away.
If you know when you conceived, it adds 266 days (38 weeks) instead, because conception usually happens about two weeks after a period starts. From a scan, it counts forward from the age the sonographer gave on the day of the scan.
Which date should you trust?
Your dating scan, usually between 11 and 14 weeks, gives the date your midwife will use. It measures the baby and can move your due date by a few days. If it differs from the date worked out from your period, go with the scan.
Pregnancy usually lasts between 37 and 42 weeks, so think of your due date as the middle of a window rather than a fixed day.
What your timeline shows
The timeline lists the antenatal care usually offered in England with the dates that apply to you: the booking appointment, the 12-week and 20-week scans, and the 16, 25, 28 and 36-week appointments. Care in Scotland, Wales and Northern Ireland follows a similar pattern. It also shows the work dates that depend on your due date: when the MATB1 certificate becomes available, the last day to tell your employer and the earliest day your maternity leave can start.
From 28 weeks, UK guidance is to go to sleep on your side, for daytime naps as well as at night. Many people find a long pregnancy pillow makes side sleeping more comfortable. Ordinary pillows can work just as well, so choose whatever feels right for you.
Questions
How accurate is a due date calculator?
It gives an estimate. The date from your dating scan between 11 and 14 weeks is more accurate, especially if your periods are irregular or you are not sure when your last period began.
My cycle is not 28 days. Does that matter?
A little. Ovulation usually happens about 14 days before the next period, so a longer cycle means later ovulation and a later due date. Choose your usual cycle length and the date moves by the difference from 28 days.
Can my due date change?
Your midwife may change it after your dating scan. After that it normally stays the same, even if later scans show a larger or smaller baby.
When does each trimester start?
The first trimester runs to the end of week 12, the second from week 13 to the end of week 27, and the third from week 28 until birth. These are the stages used in the NHS week-by-week guide.
When should I start using a pregnancy pillow?
There is no set week. Many people start in the second trimester, when the bump makes it harder to get comfortable. From 28 weeks the advice is to go to sleep on your side, and a pillow can help you feel settled there. It is a comfort choice, not a medical requirement.
Do you keep my dates?
No. The calculation runs in your browser. Your due date is saved on this device only, so our other pregnancy tools can fill it in for you.
Sources
Last reviewed 13 October 2026. These tools give estimates for information only and are not medical or legal advice. Your midwife, GP or fertility clinic confirms your dates; your employer and gov.uk confirm your leave and pay.