Due Date Calculator

How is a due date calculated?

A pregnancy is dated from the first day of your last period, not from conception. The standard method adds 280 days (40 weeks) to that date. If your cycle is usually longer or shorter than 28 days, the calculator moves the date by the difference, because ovulation tends to happen about 14 days before your next period.

Which due date will my midwife use?

The date from your dating scan. The NHS offers this scan at around 10 to 14 weeks, and it checks how many weeks pregnant you are, whether you are expecting more than one baby and that the pregnancy is in the right place. If the scan date differs from your calculation, the scan date is the one your maternity team works to. After IVF, your clinic will confirm the date from your transfer.

How accurate is a due date?

It is an estimate of the middle of a range. Most babies are born in the weeks either side of their due date, and only a small share arrive on the day itself. The NHS offers induction if labour has not started naturally by 41 weeks, so it helps to think of a due fortnight rather than a single day.

What are the key dates in pregnancy?

• 10 to 14 weeks: dating scan, with screening for Down's syndrome if you choose it.
• 13 weeks: the second trimester begins.
• 18 to 21 weeks: the mid-pregnancy screening scan (often called the 20-week scan).
• From 16 weeks: the whooping cough vaccine is offered in every pregnancy, and the RSV vaccine from 28 weeks. See our vaccination guide.
• 27 weeks: the third trimester begins.
• 37 weeks: your baby is considered full term.

Use the pregnancy week-by-week guide to see what is happening at each stage, and the trimester to-do list for the practical jobs.

When should I tell my employer?

GOV.UK says to tell your employer at least 15 weeks before your due date when the baby is due and when you want your maternity leave to start. The calculator's due date lets you work out that deadline; our maternity pay calculator estimates what you could receive.