Estimated Due Date – EDD


The actual percentage of babies born on their due date in the UK is only 4%! Can you believe that!? This seems extremely low, but rates rise rapidly on the dates surrounding the given due date. 60% of babies are born within a week either side of the estimated due date (EDD) and more than 90% are born two weeks either side of the predicted date, (data from the Perinatal Institute, a non-profit organisation). 

But where did due dates originate?

In the 1700’s Hermann Boerhaave, a professor from the Netherlands, explained how to calculate an estimated due date (EDD) on the records of only 100 pregnant women. He figured out the EDD by adding 7 days to the last period, and then adding nine months. Although he didn’t explain whether this was taken from the first or last day of the last menstrual period (LMP). 

This was then ‘updated’ by a German Obstetrician called Franz Karl Naegele in 1812 where he stated a due date would be based on adding 7 days to the LMD (from the first day of the period), subtracting 3 months and adding a year.

Both theories were based on 28 day menstrual cycles, which we know isn’t the norm for most people. Calendar months and years change too, so this then questions the accuracy of these EDD’s.

Ultrasounds came into practice in the 1970’s which helped to support EDD’s as you can now see the growth and development of the foetus in utero. This can determine how many weeks and days pregnant a person is, which is extremely helpful for screening tests and growth observations. However it still can’t predict when your baby will arrive! Each birther is unique in their genetic makeup, history, age and environment. All things that could and should be taken into consideration. 

What we need to remind you is that this is an ESTIMATED due date! It helps you to plan and prepare for the birth of your child, but according to a large study of over 30,000 pregnancies (Smith 2001*) this showed that 50% of first time mums were still pregnant 5 days past their due date (*Smith, G. C. (2001). Use of time to event analysis to estimate the normal duration of human pregnancy). 

Unfortunately EDD’s assist hospitals and medical centres to try to intervene when spontaneous birth hasn’t happened by the EDD. Extra vaginal examinations and methods of induction will more than likely be presented to birthers close to or over their EDD. 

New NICE recommendations even include offering induction at 39 weeks to birthers of black, Asian and ethnic minority backgrounds, birthers with a BMI over 30, and birthers aged over 35 years.

Without being informed of their rights, choices and understanding of how EDD’s are determined, birthers may feel that these sorts of interventions are needed and they may be ‘gently’ coerced into making decisions based on misrepresented information.

Managing expectations, understanding risks and birth rights and helping birthers to relax and not focus on a set date will help them to understand that due dates are a load of bollocks! 


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