That’s a 1 in 20 chance of a baby being born on its due date!
If you remember as far back as week 40 you may recall that we looked at how full-term was defined. We found out that “full” term differs from country to country and that term is defined, by the World Health Organisation, as anywhere between 37 and 42 weeks.
How to calculate a due date is a disputed and much-discussed topic, one that requires its own post to discuss the different methods and calculations. We’ll look at the most common and least mathematical of them all – the dating ultrasound scan (sonograph). In the UK, this usually occurs around 8-14 weeks into pregnancy and is used to give you an “estimated due date”. The key word is “estimated” because there is a tolerance or discrepancy with every scan.
For example, let’s take a look at the margin of error for the 2 scans you would expect with the NHS:
8-14 weeks. First scan (AKA the dating scan)
8-14 weeks (56-98 days) ~ 5-8 days either side of the date given. That’s a 10-16 day discrepancy.
18-21 weeks. Second scan (AKA anomaly scan, mid-pregnancy scan, is it a boy or a girl scan?)
18-21 weeks (126-147 days) ~ 10-12 day tolerance or 20-24 day discrepancy
As you can see the later into your pregnancy, the less accurate sonography becomes. There is an 8% margin of error, so the earlier a scan is, the more accurate it is. Once you get past 22 weeks into pregnancy, an ultrasound cannot be used to estimate the due date because the size no longer represents a baby’s age very well. Even average babies can vary by up to 2-3 “weeks of growth”.
There are some scans that take place at 36 weeks (252 days). If these were used to calculate a due date, they would have a 20-day tolerance and a 40-day discrepancy – that’s nearly 6 weeks!
In week 40 we asked if estimated due dates were lucky guesses or well-calculated. The answer could be BOTH. If a scan is taken close to 8 weeks then it is fairly well-calculated. Once past this early window, it becomes more like lucky guesses!
Wherever you may be in your pregnancy, don’t sweat the due date and think of it more as a due period. Once you hit 37 weeks you’re classed as term, so anytime from then until 42 weeks!