So where did this idea come from that all babies are due at 40 weeks?
Apparently, Aristotle was the first person to document that human gestation was 10 lunar months. Which was then interpreted as 280 days, because a lunar month is 28 days.
But all lunar months aren’t 28 days. So even this original assumption was flawed.
And we are not even sure if Aristotle was recording a cultural belief or a “scientific observation”.
The first person to write down an algorithm for estimating due dates was a Dutch doctor, Hermann Boerhaave, in 1709 He said that the date of birth could be estimated by adding nine months and seven days to the date of the last menstrual cycle.
Boehaave’s calculation was then reported by Frederich Naegele in 1830 and thus became known as Naegele’s Rule.
This is the calculation still used today. We now add 9 months and seven days to the first day of a woman’s last menstrual period. However, it is interesting that neither Boerhaave nor Naegele specified which day of the last menstrual period was to be used. For a while, we used the last day of the last menstrual period, but for some reason, it changed in obstetric texts to the first day of the last menstrual period.
Naegele’s rule was developed into the pregnancy wheel, which, again, is still used by some care providers today.
Over the years, research has shown that Neagele’s rule doesn’t accurately predict the average length of a pregnancy.
A 2001 study found that the median length (50% before and 50% after) was 40 weeks and 5 days, not 40 weeks
The modern-day equivalent is the “Due Date” App which is still just based on algorithms and the assumptions every woman’s body functions exactly the same way and all babies develop at exactly the same rate.
The research by Mittendorf and others suggests that we should include parity (how many babies you have had), ethnicity, height, and age, into the equation when calculating a due date. I wonder if App developers have read this research or are still using Naegele’s Rule?
And I wonder if they tell you a different due date if you are French or Japanese? Because if you were in France or Japan, your “due date” would be 41 weeks of pregnancy.
But we have an ultrasound dating now, I hear people screaming!
Yes, we do, and the research shows that they are just as good at predicting when your baby is 40 weeks as you are if you know the date of your last LMP and have regular cycles.
But is this the point?
All the above methods can do, is tell you when you are 40 weeks pregnant, to varying degrees of accuracy.
None of them can tell you when YOUR baby is ready to be born.
Only your baby knows that.
I go back to my point about children all developing at different rates.
Why don’t we think our babies develop at different rates in the womb?
Just because two babies are both 38 weeks doesn’t mean they have developed in exactly the same way.
In YOUR baby’s last weeks of gestation, amazing things happen including:
antibodies are passed from you to your baby to help him/her fight infections in the first days and weeks of life
brain development speeds up
suck and swallow coordination improves
brown fat increases meaning your baby will be able to regulate his/her temperature
iron stores increase
But remember, we have no way of telling when this will happen for YOUR baby.
Wouldn’t it be better to just wait until YOUR baby tells you that he or she is ready to be born?
I have so much more to say on this topic as I know many of my clients are being induced because they reached a certain “date”, not for any clear medical reasons.
In 2020 the website Every Week Counts was launched.
Through the linkage of very large population health records, we are able to identify the importance of the last few weeks of pregnancy and the growth and development that takes place between 34 and 40 weeks of gestation. These findings have led us to promote evidence based best practice for planned early birth across New South Wales and Australia. (www.everyweekcount.com.au/about-us)
The main importance was to do with brain growth in the last weeks of pregnancy
As research in neuroscience and imaging advances, we are able to see babies’ brains more clearly – even before they are born. This is improving our understanding of the brain and how it develops during the last few weeks of pregnancy. (www.everyweekcount.com.au)
This has led to a campaign for redefining term and the statement that no baby should be born before 40 weeks without clear medical reasons.
I know that there are some care providers who think there are medical reasons for inducing your labour just because you have reached a certain date. However, the evidence for this is not robust and I will share this in subsequent blogs.
In the meantime, read these very thought-provoking articles:
