The ridiculous origins of your due date!

Have you ever questioned the accuracy of your due date?

It might not be something you’ve thought about yet, but when you get to the end of your pregnancy, you may suddenly find that a lot of importance is placed on your due date, so it is worth understanding how it is calculated, and how it can affect your birth.

Due Date

I have recently undertaken a fascinating course in this subject and the overall message I have for you, is that there is no evidence backing up the 42 week rule. That is because women are all different, our pregnancies are all different and our babies are all different!!!

Post term pregnancy

It is really important that you are aware of the ridiculous origins of your due date, because once you reach 40 weeks (your due date), you are immediately treated like a ticking time bomb. A lot of women are offered sweeps at 40 weeks (and then at every subsequent appointment). And a lot of women are automatically booked in for induction of labour at 42 weeks. Unfortunately, there isn’t always a proper discussion about this, and what it can mean for you, your birth and your baby.

So a lot of intervention can occur purely based on this due date…..but what if this due date was totally inaccurate, dates back to Ancient Greece, is based on the stars, and has no evidence to back it up?!

Well that is the frustrating truth in all of this.

The current method used to calculate your due date is based on a formula called Naegele’s rule. This takes the first day of your last menstrual period (LMP) and adds 280 days to it.

Variations in menstrual cycles
As you already know, our cycle lengths are all different, so if you have a longer cycle than average, you are likely to ovulate, and therefore conceive, later in the cycle. But this is rarely taken into account when you are given your due date. In fact, in a recent study among natural conceptions where the date of ovulation was known, the variation in pregnancy length (even after excluding preterm and complications), was 37 days. That is a huge variation in length and shows that it is absolutely ‘normal’ to have a longer pregnancy.

And where does the 280 days come from?
Well Aristotle was the first person to document that the average pregnancy was 10 lunar months which was then interpreted as 280 days (because a lunar month is 28 days from a fixed point in space). So we’re already on slightly shaky ground because Aristotle wasn’t in a fixed point in space, he was on earth, and lunar months vary depending where on earth you are.

To add even more scepticism to this arbitrary number, there are a couple of researchers who think that Aristotle may have picked this number based on what was astrologically significant in ancient Greece. So it is essentially a number based on the stars!


When you do look at the average length of pregnancy in real women, you find that it is more than the 280 days used in the current calculations, more like 283-288 days.

Research has shown that a woman’s height, age and ethnicity can all influence her baby’s gestational length.

It therefore seems crazy to me that so many women are being induced or told they might need to be induced, based purely on their 'due date', which has been calculated using observations from the Ancient Greeks and takes no account for individuality.
How can we all be treated with broad sweeping policies?

You deserve to be treated as an individual, to be provided with the information to make an informed decision, to trust in your body and your baby.

I work with my clients to help you understand your options, point you in the direction of up to date research, and help you to stay in control of the decisions around your birth.

I run monthly hypnobirthing courses in Cardiff. I also offer private sessions in the comfort of your own home, which can be completely personalised to your circumstances and arranged at a time to suit you.
Included in all my courses are 4 hypnobirthing downloads so you can practice at home all the way up to your birth, a fantastic resource pack with all the important information you need, and email or phone support from me right up until your birth.

Don’t forget to sign up to my newsletter to receive a copy of my ‘Top Tips to prepare for a calm and confident birth’, and have a look at my website for more details on the services I provide.

I look forward to helping you prepare for your calm and confident birth!