How accurate is my due date? 3 key facts about due dates you need to know
how accurate is my due date from 12 week scan

Decoding Your Due Date: how accurate is my due date?

So you have missed a period (or maybe you haven’t even missed your period yet), and you have taken a pregnancy test and seen those magical pink lines, blue cross or beautiful work ‘pregnant’. But where do you go from there? How far along in your pregnancy are you? And what is your due date?

These seem like simple questions, but unfortunately not all is as simple as it seems and even with all the different ways of calculating your due date you need to question, just how accurate is my due date?


Let’s start at the beginning… what is a due date and how is it calculated?

An Estimated Due Date is when your medical professional predicts you will be 40 weeks pregnant, know as ‘full term’ and when your baby is considered ‘due’. However, this is complicated from the off. The idea of being 40 weeks pregnant is calculated as 40 weeks from the first day of your last menstrual cycle (your period), but this is based on the idea of a textbook 28 day long menstrual cycle where you have ovulated on cycle day 14. However, this is surprisingly uncommon and although most women ovulate between cycle day 12 and 16 if you have very long, irregular cycles or conditions such as PCOS your ovulation could be wildly different from that ideal cycle day 14. I personally have PCOS and have ovulated and conceived on the 90th day of my cycle so that kind of calculation would have been incredibly useless for me! The work around for this is adding 38 weeks to the day of ovulation, but this only works if you are tracking ovulation as part of trying to conceive. So already, the due date you have from typing dates into an online calculator is already likely to be wrong by a few days, or even several weeks!

Is pregnancy even 40 weeks?

Even the idea of pregnancy being 40 weeks long is an idea from back in the days of the Ancient Greeks who used lunar cycles to predict due dates and finding that women’s pregnancies typically lasts 10 lunar cycles (of 28 days), giving us 40 weeks. However, a study in 2011 looked at over 1500 women, gave them very early (around 6 weeks) ultrasounds which is viewed to be the most accurate way of indicating how far along a pregnancy is, and found that on average women gave birth at 40 weeks and 5 days (this was then replicated in another study in 2013, finding the exact same). If you were living in France your due date would actually be at 41 weeks, not 40 weeks as it is in the UK. Studies have also found that women only have 5% chance of giving birth on their due date!

Why might my due date change?

The 12 week scan is also know as the Dating Scan, and at this ultrasound the sonographer will take a measurement called the Crown Rump Lenght (CRL) which is the measure of the distance between your babies bottom and the top of their head. However, this still tricky to get completely accurate as babies at 12 weeks are surprisingly wriggly around and are also notorious for curling up into positions that make getting measurements tricky. When you consider that a baby at 12 weeks is just 5.4cm a measurement being out by just a couple of mm could change the due date by several dates, and this CRL measurement to predict gestation also doesn’t take into account factors such as is this baby likely to be particularly large or petite because of height and build of parents, and whether the mother smokes (which is provent to result in babies smaller than normal for the gestation). All of these factors combined it is easy to see how the due date calculated from an ultrasound is just as estimated by ultrasound as it is by calculating from your menstrual cycle.

That said, if this date calculated at your 12 week scan is more than a day different to that calculated by your period then your offical due date on your medical notes will be changed to reflect the date calculated at your ultrasound.

How accurate is my due date – why does it matter?

We all know that babies come when they want and we have proven above that the due date is often inaccurate but why should we care? Well it all becomes more complicated towards the end of your pregnancy, as in line with current NICE guidelines, an induction for being overdue is now offered at 41 weeks of pregnancy, just 7 days after your ‘due date’ and only 2 days after studies have shown to be the average lenght of pregnancy and when we consider that the due date could easily be inaccurate by several days this puts the liklihood of you reaching that 41 week mark more likely. Induction is a huge topic so look out for other blog posts about this but I highly recommened looking at the work of Dr Sara Wickham to find out more about why induction being recommended simply for being 41 weeks pregnant is a complicated and nouanced decision that every family should take into careful consideration, remembering that their due date is unlikely to be perfect.


And finally, one piece of advice around your due date, remember it is only an estimate anyway…. give your friends, families and loved ones a slightly later due date that your offical estimated due date because in those final weeks and days of your pregnancy the daily texts and phonecalls asking if there is any sign of baby yet might just drive you insane and if they don’t know your actual due date then they won’t be able to do that.

Join hypnobirthing courses in Surrey that help you practice a calm birth

Book a course

Fill in your details below to register your interest in an upcoming hypnobirthing or breastfeeding course. I will be in touch with 24hours to complete your course booking. Please note that you place on a course is not confirmed untiI a deposit has been arranged. 

Contact Form