Are due dates accurate?

Written By Emily Hanton

The short answer is no, due dates are not accurate. Read on to find out why, what you can do instead and why it all matters.

Can due dates be wrong?

YES. There is only a 5% chance that your baby will be born on your due date; that means there’s a whopping 95% chance that your baby will NOT arrive on the date you have been given. So go ahead and plan a date night, a meal with friends, or just a night snuggled up with a book – because it’s very likely that you’ll be able see it through.

How are due dates calculated?

I’m afraid to answer this well, we travel back a good few centuries…prepare to be shocked! The first records were written by Aristotle almost two and a half thousand years ago; he wrote that pregnancies last approximately 10 lunar months.

Fast forward to the early 1700’s, a doctor called Hermann Boerhaave updated this slightly and proposed a way of working out a due date – add 7 days and 9 months to a woman’s last period. It must be said that this was not based on good, solid research. Nevertheless, this was later reported by Franz Carl Naegele and is known today as Naegele’s rule.

When you visit the midwife at your initial appointment, the due date you are given is based on this rule.

Research has shown that the rule is not an accurate reflection of pregnancy duration and that there is lots of variation between women. Consider for a moment the variation in the length of menstrual cycles – some people may have a cycle that lasts 28 days, whilst for others it may last 35 days – it should be said that both of these are normal, just DIFFERENT. Just as there is variation in menstrual cycles there are also variations in pregnancy duration (again this is normal, just DIFFERENT). It makes no sense to have such a rigid rule specified down to the day, which is applied to every single mother.

To evidence this point, research conducted by Jukic et al. (2013) found that even when the exact date of conception is known the variation in pregnancy length is 37 days – just over 5 weeks!

The belief that a fixed and very standardised rule can be applied to everyone is a very simplistic approach and doesn’t account for the intricacies and complexities of the amazing human body. Lawson (2020) revisited Naegeles rule and found that several factors influence the length of a pregnancy; this included ethnicity, height, variations in menstrual cycles, timing of ovulation, number of times a woman has given birth and the mothers weight.

 Why does the due date change?

As mentioned above, your midwife will use Naegeles rule to calculate your due date during your first appointment, but when you have your 12-week ultrasound scan you may be given a different date.

Which due date is more accurate?

Ultrasound is often promoted as a more accurate way to estimate due dates. It uses a very different approach to Naegeles rule, it’s based on the assumption that there is a link between the size of the foetus and the length of pregnancy. So, your midwife will look at the measurements taken during the scan and plot it against the measurements on their chart which then gives a due date. This method ignores the fact that normality is a range and that there are differences in growth rate between foetuses. What if the day before the scan the foetus has a little growth spurt – there is no way of knowing this and the results on the day of the scan are the ones used as the predictor of the due date. We must keep in mind that we are talking about tiny measurements on a screen that are being picked up through a person’s stomach.

Whilst ultrasound scans are slightly more accurate than due dates predicted using Naegeles rune, the reality is that for two thirds of women the 12-week ultrasound can predict a two-week window within which their baby will be born and cannot reliably predict one specific day (Khambalia et al. (2013)). Furthermore, one third of women will give birth outside of this window and there is no way of knowing which women will give birth within this timeframe and which ones won’t.

What can I do instead?

It’s really useful to understand that your estimated due date is a best guess, with a great deal of wiggle room. It’s unfortunate that due dates have been handed so much power as it can cause lots of hype around going “over-due” as soon as you go past this date, even though it is completely normal for babies to be born anywhere between 37 and 42 weeks of gestation. The truth is that you have a huge five-week window where you may go into labour.

So instead of a due date, why not give yourself a due month! Even better, if you haven’t told anyone the due date you have been given…don’t…why not tell them your due month instead? So that if your due date comes and goes, with no sign of baby (as it does for many women) you won’t be bombarded with people questioning if you have given birth yet – you don’t need that kind of pressure and stress at that time! Stress and labour DO NOT go hand in hand!

 Why does all of this matter?    

Due dates are treated as a date by which you should have given birth, as though it’s an expiration date and the research shows that this just isn’t the case. The due date is handed an enormous amount of power and there can be a lot of pressure to give birth close to your due date, from family and friends who are eagerly awaiting baby’s arrival to health care professionals who have guidelines to follow. It’s worth noting that just because there are a set of hospital guidelines (which vary between hospitals) it doesn’t mean that you can’t make your own decisions – remember you are the one in control and you can decide what is best for you and your baby.

Going ‘over-due’ will often lead to interventions designed to initiate labour - there can be benefits but there are also consequences and risks involved in choosing interventions (such as induction) so it’s worth doing some research before making a decision.

Feeling pressured to go into labour can cause stress which releases hormones that actually delay the onset of labour. Feeling calm and relaxed is one of the most important things during pregnancy and birth; learning a range of techniques to feel this way will be extremely valuable - the importance of this should not be under-rated.

My favourite birth affirmation: Your baby will come when your baby is ready.

A baby doesn’t need someone on the outside looking at a calendar and deciding today is the day it should be born. A woman’s body knows exactly how to grow a baby; creating all of its little organs, limbs, and its tiny fingers and toes. When the time is right and baby is ready for the outside world it sends a signal to its mother and the two of them will work together for baby to be born. Our bodies and our babies know what to do, if we let them get on with it.

 

To sum up, due dates are NOT accurate, they are a best guess. Rather than working towards a specific date, give yourself the luxury of a due month and take away some of the stress and pressure. Don’t forget – your baby will come when your baby is ready!

 

What next?

Welcome to Bloom in Birth.

I'm Emily, a Hypnobirthing Specialist and Antenatal Educator who is passionate about helping women to have a positive birth experience. I offer transformative birth preparation courses that help you to feel calm, confident and capable as you approach labour.

Find out more about me here.

Emily Hanton
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