Cooking Times May Vary (by 5 Weeks), or the Curse of the EDD

by Oct 8, 2013

Ah, the infinite variety of bread!

I love the way you can alter the ingredients just a tiny amount and get something completely different. And you never know how long it will take to prove, or how large it will get. Then stick it in the oven and depending on how much kneading, the type of ingredients you use, their ratio, and the temperature of the oven, it always results in a totally different loaf. And for each loaf the cooking times will vary according to your oven.

And yet compare this relatively simple concept with, say, human gestation and we start to see our obsession with an “estimated due date” as a bit daft! Or possibly even totally ludicrous!

Studies published in 2013 show that a “normal” pregnancy can be reasonably considered full term anything from 37 – 42 weeks! [1] While we don’t seem overly concerned with a baby that arrives in the first 3 weeks of that window, for some reason we have come to believe that our bodies will somehow know they have reached that magic 40 weeks date and will stop cooking the baby it has carefully nurtured for the previous 9 months. As soon as we hit that last two weeks, we become all too ready to opt for an induction of labour to give the baby a bit of a “helping hand” in their journey to the outside world.

There are some risks after 40 weeks, and I will be discussing them here soon, but first let’s look at how we have come to the “estimated due date”.

Pregnancy due dates are calculated using Naegele’s Rule [2]: “The rule estimates the expected date of delivery (EDD) by adding one year, subtracting three months, and adding seven days to the first day of a woman’s last menstrual period (LMP). The result is approximately 280 days (40 weeks) from the LMP.”

Most NHS midwives have a clever little wheel that allows them to quickly and easily work out your due date, and how many weeks into your pregnancy you are. It is genius really!

Except it is not quite right, is it?

It assumes that every woman has a completely regular, 28-day menstrual cycle. Wrong. Do I need to elaborate? I think not. So this alone is going to throw out most calculations. Nevertheless, assuming we remove the two weeks added in for the menstrual cycle to conception, that leaves us with 266 days from conception to EDD and many online calculators will make allowances for the length of cycle [3]. NHS midwives still like the wheel though, and we still end up with a single EDD.

We also have a dating scan at 12 weeks to verify the due date, but if it falls within a week either side then the EDD doesn’t change.

In addition, our interpretation of Naegele’s Rule assumes that all babies cook for a particular amount of time and there is one day that is the most likely one that a baby will appear. This, of course, is what the studies undermine but it is very hard not to need to have a specific date in our heads that our baby is likely to come for reasons of both a practical and emotional nature. How can we prepare if we have a 5-week window?

Finally, it is perhaps worth noting Naegele’s birth year. And death year. 1778–1851. One might reasonably assume that the health and well-being of German women at the turn of the 18th/19th centuries might be somewhat different to today’s modern woman. (his rule was published in 1806.)

So it is no surprise really that only 4% of babies hit their EDD, and only 70% within 10 days of it. [1] That means nearly a third of all babies are not born within 10 days of their due date!

Further studies have shown that Naegele’s rule is a fairly good one actually, [2 and 4] but most seem to indicate that a more accurate average gestation length is slightly above EDD – anything up to a week, really. We probably need to be a bit more relaxed about the whole thing or even follow the French method of stating 41 weeks as the due date. [5]

But how would you feel at your booking in appointment, if your midwife said, “Congratulations, your baby will be due sometime between mid-February and the end of March. Off you go, dear!”? (Apologies for implying midwives are patronising, actually, they are usually lovely and wonderful people.)

We have to have an EDD – although I wonder if there is scope for updating Naegele’s rule somewhat – but try very hard not to fixate on it, and to try to think of it as the mid-point of a window, or at the very least not start to panic that something might be wrong until the window is nearly over, rather than the moment we get past the first 3 weeks.

And certainly, if we go over the magic 266 days, that we keep the faith that our ovens have their own thermostats, and brilliantly clever sensors that know exactly when the bread is ready and will pop out of the oven just cooked to perfection!

Active Birth teaches that we have to trust our body to birth the way it needs to, and so we should extend this to awaiting our due date, and let it pass with relaxation and not fear, knowing that our bodies know exactly what they are doing.

Learn more about Active Birth here.



[1] Albeit not the original research this is an easy to read digested version.


[2] Naegele’s Rule, according to Wikipedia[3] Here is a nice online calculator for due date that uses a variety of methods to come up with a date.
[4] A nice summary of studies of the most likely date babies arrive.[5] How to calculate your due date the French way



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