due date calculation

Your due date is wrong!

Yep! Well, it might be. In fact, more than likely.

How do I know this? Because the EDD – Estimated Due Date – is one of the biggest [lies/disasters/ trick /hoaxes etc] to affect pregnant women today. And, even though people who should know better (doctors, midwives etc), know better, this lie still stands. It’s a bit like the whole Santa thing, but far more damaging.

WHAT? What? Tell me!!!

And because of this, you’re setting yourself up for a ton of stress in the last month or so of your pregnancy.

So why is it wrong?

The most commonly used method to calculate due dates is also the most inaccurate. Seriously. Let me explain.

The due date system that is widely used by doctors and midwives is based on Naegele’s Rule (Franz Karl Naegele OB, 1778–1851).

Naegele’s Rule

According to Naegele’s Rule, the standard definition for gestational term is 266 days from conception to the date of the baby’s birth. This is also defined as 280 days, or 40 weeks, from the first day of the mother’s last menstrual period, a definition which assumes that the mother ovulates on day 14 of a 28 day menstrual cycle. The actual formula used to calculate estimated due date (EDD) is:

(LMP + 7 days) – 3 months = Due Date

This is the formula that was probably used to give you your due date.

In fact, Naegele’s theory originated from a guy called Harmanni Boerhaave, a botanist who in 1744 came up with a method of calculating the Estimated Due Date (EDD) based upon evidence in the Bible that human gestation lasts approximately 10 lunar months.

I don’t know about you, but for me, there is so much wrong with that last sentence, that when I first read it my jaw bruised my big toe. Considering what’s at stake, this calculation method doesn’t exactly inspire me with confidence… and here’s why:

The Bible?

Like HOW long ago?! You would think that given how many people have been born since then, that we would have built up a pretty good picture of this whole human gestation thing. You know maybe, even taken the time to take a closer look to see if the assumptions we’re using are appropriate…. You’d think?!

“…based on evidence in the Bible…”

Since when is the Bible a reliable source of evidence? Or indeed a reliable authority on the passage of time. The Bible said that the Earth was created in seven days, and yet most of us have accepted that that is highly unlikely, if not downright impossible. But yet here we are using an idea from the Bible to tell women when they can expect to bring their mini human into the world. What makes this unforgivable in my mind is that this rule is NOT based on scientific or empirical evidence or research. Yes, you read that right… it is NOT based on empirical evidence or research. None whatsoever. Not even 100 year old scientific research when hospitals were a lot muckier than they are now. No! We’re talking about times when women gave birth along side cattle and donkeys in barns by candlelight.

Can you tell that this makes me mad? But, it gets worse…

“…that human gestation lasts approximately 10 lunar months”

Lunar cycles?!! Seriously? OK.. Well let’s follow this line of thought and see where it takes us.
A quick check online will tell you that there are five different lunar months ranging in length from 27.3 days to 29.5 days, but the one that is widely used is the synodic month, which wikipedia describes as “how long it takes on average to pass through each phase (new, half, full moon) and back again” and which lasts 29.5 days. So if the Bible assumes that human gestation lasts approximately 10 lunar months then according to my calculations 10 x 29.5 = 295 days.

Now if you recall, Naegele’s Rule says that human gestation is 280 days. So, given that 10 lunar months gives us 295 days… we’re already hitting some problems… Let’s do some more maths..

295 – 280 = 15 days difference

This is 15 days LONGER than the 280 days gestation period that is being used and that we’ve been lead to believe is average.  That’s 2 weeks! So, even the rule is confused!

Already we can see that using Naegele’s Rule, gives us 2 week slacks when it comes to the due date calculation.

But hang on, I’ve not finished…

28-day menstrual cycle

Naegele’s Rule is based on the idea that our menstrual cycle lasts 28 days. Hands up – whose cycle DOES NOT last 28 days? Most of room, huh? Thought so… We all know that our menstrual cycles vary in length… I know mine is nearer 34 days than 28 days. Do you know what yours is? It’s accepted that menstrual cycles can last anything from 22 to 35 days. So if your cycle is NOT 28 days then you need to tweak the formula. If your cycle is 34 days, then you would need to add 6 days to your due date (34 – 28 = 6), whereas if your cycle is 26 days, you would take 2 days off (28 – 26 = 2).


And finally, Naegele’s Rule assumes that we ovulate exactly half way through our 28-day cycle … 14 days in. But ovulation is not always half way through a cycle as it can be affected by things like whether you’ve recently come off the pill, or if you’re stressed, ill or experiencing a disruption of routine. In other words; LIFE!

Now that we’ve peeked behind the curtain that is Naegele’s rule…. What is your level of confidence in the due date calcualtion that you’ve been given as your Estimated Due Date?

Isn’t it time to find a date that is more realistic? And perhaps one that is based on a method that is backed by science?

The 3 alternative due date calculation methods

During the podcast I said that I’d share with you 3 other due date calculation methods that are based on scientific research carried out in the twentieth century. But before you can work out a more realistic due date using these methods, you’ll need to be armed with certain pieces of information. You will need;
1.    The start date of your last menstrual period (LMP)
2.    Your average cycle length (in days)
3.    What number pregnancy you’re on… a pregnancy being defined as ‘A woman who has given birth to an infant, liveborn or not, weighing 500g or more, or having an estimated length of gestation of at least 20 weeks.’

The 3 alternative due date calculation methods are based on the work of

  • Park (1968)
  • Nichols (1985a)
  • Mittendorf et al (1990)

Now during the podcast I mentioned that I would share with you a cheat sheet with all of the formulas from each of these due date calculation methods. The cheat sheet contains the formulas for each of the alternative due date calculation methods so that all you need to do is to work it out.

To get hold of that cheat sheet simply click on the button below. When you sign up below you will also receive emails to help you to feel calm and confident as you approach your birth.

due date calculation


Resources & Further Information

During the podcast I mentioned that I would share with you my sources of information and research, well here they are..

Postdates: Separating fact from fiction

My favourite article as it’s nice and detailed but clear and easy to read:

Calculating Due Dates and the Impact of Mistaken Estimates of Gestational Age

The lie of the EDD

A timely birth – Midwifery Today

This is a fantastic and very thorough article on due dates, inductions, risks of waiting etc and includes all the latest scientific, evidence-based research.

Did you work out your alternative due dates? How different are they? Has knowing this changed how you might plan the last month of your pregnancy, like when to stop work? What have you decided to do in terms of telling friends and family about your due date? Let me know in the comments below!


Alexia Leachman
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