Every woman and every pregnancy is different. The average length of time a pregnancy lasts is around nine months, but there is a wide range of dates within which labour can start within a healthy pregnancy.

It is very useful to work out your birth month as this helps you get prepared and remain confident if you have a longer pregnancy. Find out how to work out your due date below.

When is my baby due?

A due date or estimated date of delivery (EDD), is needed for your NHS maternity care. Routine checks are organised around this date and will be considered when offering any treatment you or your baby might need. However, only five per cent of babies arrive on their EDD so the idea of a due date can be unhelpful.

At the beginning of pregnancy, you will be asked for your last menstrual period (LMP) date. This is the date when your most recent menstrual period started. This data is used to generate an approximate EDD and is then adjusted after measuring the baby at an ultrasound scan at around 10 to 14 weeks into the pregnancy.

If you don’t know the first day of your last period, don’t worry; you can give a best guess. The important thing is to self-refer now so you don’t miss out on care.

Your midwife or doctor will talk about the length of your pregnancy and the gestational age of your unborn baby in weeks. Your pregnancy is described as reaching 40 weeks on your estimated delivery date. This might also be written as 40+0 which means forty weeks exactly. The day after can be written as 40+1, and so on up to 40+6 before the next week starts. 

Use the NHS Due date calculator to help you work out your due date.

Your birth month runs from three weeks before your due date, until two weeks after your due date. (Your birth month is 35 days, so a bit longer than a calendar month). It is normal for labour to start at any time in this range.

Research shows that using ultrasound dates reduces unnecessary intervention towards the end of pregnancy. If your pregnancy reaches the last week of your birth month, then you can discuss both date estimates with your midwife or doctor to develop a personalised plan.

On average, labour starts a few days after the EDD for women who have had a baby before, and about a week after the EDD for a first baby.