Pretty much as soon as you find out you’re pregnant you’re on google trying to work out when your baby will arrive (no judgement, been there ), this will be confirmed at your booking in appointment based on your last menstrual period date (it might be changed again if you have a dating scan but we’ll come to that another day).
But how do we get to the due date? Here in the UK the way we calculate the estimated due date (EDD) using the principles of Naegele’s formula where we add 7 days to the LMP, take off 3 months, then add a year. Or, add 280 days to the LMP. Simple
This formula was first published in the 1700s and was based on the biblical reference of pregnancy lasting 10 lunar months. So y’know, accurate and evidence-based. OH WAIT. Boerhaave, who FIRST came up with this, helpfully published his calculations with the description ‘count one week from the last period’ without specifying whether this was the beginning or the end of the period. Thanks for the help, Boerhaave, didn’t your teacher ever tell you to show your workings?
Naegele came along in the 1800s and ballsed up the calculation again by not clarifying what conception period he was talking about so someone else finally came along and actually laid out the workings – and added 7 days to the end of the period in the formula. Thank you Professor Bedford.
This probably explains why France’s EDDs are 41 weeks not 40 weeks. Not that French uteruses are intrinsically different to British ones.
Aside from the lack of clarity from the authors of the formula, what it also doesn’t do is take into account the length of your cycle – we know women don’t always ovulate on day 14 and we know not everyone has 28 day cycles. Eek, another flaw in the plan.
But why does it matter? Well, in theory it doesn’t because in the wise words of Call The Midwife (see image), babies will be in utero until they’re ready to not be.
And yet.
Yet when you start nearing your 40 week date the world and his wife plus every Tom, Dick and till operator will start quizzing you on “isn’t your baby here YET?!” (Yes Janet, I just have a cushion shoved up my jumper so I can be harassed by people like you every day) and your mom will start texting irritatingly often.
Not to mention that you’ll start getting pressure from your medical team to induce rather than wait for spontaneous labour.
And because you’ve had that arbitrary date in your head for 40 weeks you’ll be REALLY PISSED OFF when it comes and goes without a baby. I’ve had a +19 baby, believe.
If you’re nearing that 40 week date take a breath, book yourself some nice treats, go to the cinema alone a few times (and if you spill your bucket sized Sprite you could always try saying your waters broke for free tickets?) and enjoy those last few precious moments before your world changes beyond compare.
For the technical stuff: https://obgyn.onlinelibrary.wiley.com/…/j.1471-0528.2000.tb…
