Are you due yet? How long have you got left? What are your thoughts about ‘due dates’?

Estimated Date of Delivery or the ‘Due Date’

It’s possibly the most frequently asked question throughout pregnancy, ‘when are you due?’ “How long have you got left?’ And by week 20 you’re going to have heard it a hundred times from friends, family, the lady in the shop, the postman and other genuine well wishers. 

In medical terms EDD stands for estimated date of delivery, which, to many, is actually a better and more flexible term than ‘due date’. We all know an estimate is just that, an estimate. 

The average pregnancy lasts between 38 and 42 weeks. Setting a single date leaves only a slim 4% chance of your baby making an appearance on that specific day. We think the term EDD is a much more relaxed, less stressful term, to know your baby will arrive around a date rather than should be here on a specific date.

How do you calculate the EDD?

The ‘due date’ is simply the day your pregnancy reaches 280 days or 40 weeks, which is calculated from the first day of your LMP (Last menstrual period). If you are unsure of this information, the EDD is calculated by measuring the size of the foetus. The first EDD given can also be changed at the first scan if the foetus seems to be bigger or smaller than expected. 

The standard way of calculating the above is to use Naegele’s rule. This is named after Franz Karl Naegele who was a German obstetrician born in 1778. This formula does not always result in 280 days as calendar months differ in length and the formula does not account for leap years.

Here’s the formula to calculate your EDD using Naegele’s rule :

Date of Last Menstrual Period + 7 Days + 9 Calendar Months = Date of Estimated Date of Delivery

There is also a paper pregnancy wheel that is accurate to within 7 days. This is how your midwife would have calculated your EDD before computers were so readily available.

In modern times, a computer often calculates the EDD using a consistent 280 days since your LMP. It ‘knows’ about leap years and can work with menstrual cycles that aren’t the usual 28 days. Here’s a link to the one on the NHS website. 

What do you think about the term ‘due date’?