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Why your Estimated Due Date is wrong

Black and white image of a pregnant belly, hand on top of it

Yep, if that date you've been fixating on since you found out you were pregnant was calculated based on the first day of your last menstrual period (LMP) then it's probably not going to be your baby's birthday...

Way back in 1744, based on just 100 women, Dr Naegele suggested that the end of pregnancy could be calculated by adding 7 days to the date of the LMP and then adding 9 months (280 days). Only thing is, he never said if the 7 days should be added to start or end of the LMP. For a long time it was the end of the LMP that was used, but some time around the 1900s it changed and all of a sudden babies started being born late...

DID YOU KNOW: In France, babies are "always early"? Mais oui, they still use the last day of the LMP, so pregnancy is 41, not 40 weeks long. Less pressure, I'm into it.

OK, why are we even having this convo? Look everyone loves a countdown and some amongst us like to be organised and be prepared, so a Due Date gives everyone something to work towards but a study in 2001 (of 1514 women) showed that with most first-time mums:

50% gave birth by 40+5 weeks and 75% gave birth by 41+2 weeks

And for mums who'd given birth before:

50% gave birth by 40+3 weeks and 75% gave birth by 41 weeks

Due dates based on ultrasounds around 11-14 weeks have proven to be more accurate than using the LMP (not so surprising) but still aren't accurate. Babies are considered "full term" from 37 weeks UP TO 42! That's a long time!! 35 days!!!

Babies aren't library books, or tax bills to be paid. Their due date is just an estimate, a guess. Only 4% of babies are actually born on their due date!* There are a number of factors that influence how long a pregnancy lasts for, so there shouldn't be a hard and fast rule that is applied to everyone.

TOP TIP: add 2 weeks to the date you've been given. Practice saying that new date until you've all but forgotten the first one. Then start telling people your "due time", like "beginning of December" if you're due in the last week of November. Or, sometime in January if you're due in the middle of the month. Trust me, you can save yourself a whole heap of stress, anxiety and pressure (and irritating requests for updates from anyone who knows you're pregnant). Let go of that date and you'll be able to let go of a lot of unnecessary pressure that starts building the closer you get to that date. It's tension your body and your baby don't need as you're (both) preparing to birth.

Due dates become an issue though, when care providers want to schedule an induction if you go past 40 weeks (or even sooner!) - make sure you're asking why they're offering an induction to make sure you're not just doing it for convenience sake. Going past 40 weeks doesn't make you late. If you were in France you'd still be early! Have a read of these great articles on the Evidence Based Birth website - about Due Dates and about Inducing for Due Dates.

Doing independent childbirth education is a great way to make sure you get access to evidence-based suggestions to take into consideration as well as the knowledge you need to help you understand your body better. Plus we have loads of tips for how to get things moving along in more natural ways...

* My first son was born at 41+5 and my second was actually born on his due date!!! He arrived spontaneously but was looking to be induced that day anyway...


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