Just How Accurate is your Due Date?

Did you know that only 4% of babies actually arrive on their due date?

AND that how many weeks a pregnancy is counted as varies depending on which country you live in? In the UK pregnancy is 40 weeks, however in France it is counted as 41 weeks. so in France you'd get a 'due date' one week later than if you live a few miles east in the UK!

So if 96% of babies arrive on a different date, and the length of a pregnancy is counted differently depending on which country you live in - just how reliable is your 'due date' and why does it matter?

Where does the Due date come from?

The way we define due dates has changed very little from a calculation suggested by Franz Karl Naegel over 200 years ago. But this calculation makes a lot of assumptions! It assumes that a pregnancy will last 266 days from ovulation which he derives from another assumptiom that everyone’s menstrual cycle lasts 28 days and that ovulation occurs exactly on the 14th day. We know this isn’t the case, there is natural variation from person to person.

Ultrasound scans can also be used to estimate due dates as early cell generation occurs at the same speed for all foetuses until 6-8 weeks when their growth rate becomes unique to them. However, even with this more modern technology we are still only getting it right 4% of the time and delivery within 7 days of this date only occurs in 55% of cases.

Why does it matter?

Well, it matters because your due date is used to determine your pregnancy milestones such as scans, tests, whether the results of these tests are deemed normal, whether your baby is growing at the expected rate and decisions about when/if your baby should be induced.

Going ‘overdue’ is the most common reasons given for inducing labour. Having an induced labour can be a big deal as it comes with a number of it’s own risks and so it is important to know if, for you, the benefits of being induced outweigh the risks involved!

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