How is my due date calculated?

The date you are due to give birth becomes important as your pregnancy progresses, but how is it worked out?


One of your very first questions – probably while you’re still clutching that positive test – is likely to be “When’s my baby due?”


When you visit your GP, you can ask them to confirm your due date for you. 

How is my due date calculated?

Your due date is known as your estimated delivery date, or EDD in medical notes.

The calculation for an EDD is based on the idea that conception takes place two weeks after the first day of your last menstrual period (LMP). As pregnancy lasts around 266 days, this is added to the two weeks to make a total of 280 days, or 40 weeks, from the first day of your LMP.

The flaw in calculating an EDD is that not all women have 28-day menstrual cycles, and you could conceive in months consisting of up to 31 days. In fact, only about 5% of babies arrive on their EDD.


Mum’s story

“I suddenly realised we were having a summer baby”

“I got a scan at eight weeks because I was bleeding and my GP wanted to check everything was OK. The scan showed that my baby’s heart was beating away so that was really comforting. I also saw the midwife for the first time and she told me my due date – we’re were going to have a summer baby!”
Sienna, 32, Joe, two months


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