Almost every pregnant woman in the western world is given an estimated due date.
This is usually the date on which her pregnancy will be deemed to have reached 40 weeks in length.
But due dates are more problematic than many people realise.
What’s the problem?
We know from research studies that only 4% of babies will be born on their estimated due date.
Many people have noted that our methods of calculating estimated due dates are crude.
We also have good evidence that there is significant individual variation in the length of healthy human pregnancy.
Despite all of this evidence, which shows that a more individualised and flexible approach would be better, the focus on the estimated due date continues.
Where it all begins
Nowadays, many women calculate their own date almost as soon as they realise they are pregnant. There are many calendars or apps that can do this simply using menstrual cycle dates. Sometimes, women know exactly when they conceived, without even needing to input their cycle data.
However, these dates are often overridden, and quickly. If a woman decides to have a dating scan and the sonographer’s measurements indicate that the size of their baby is suggestive of a different estimated date of birth, for instance.
The problem with the dating scan
But this is also problematic. Estimated due dates generated by ultrasound are based on the notion that there is a link between the size of the baby and the length of pregnancy.
We know from studies that this isn’t true (Kullinger et al 2016). (See my book In Your Own Time for more on this).
Instead, there is variance in the growth of babies in utero, just as there is variance in the growth and size of two year-olds, five year-olds or sixty-seven year-olds.
Who does this affect the most?
The authors of a study published in Acta Obstetricia et Gynecologica Scandinavica set out to discover which women were most likely to be affected by this discrepancy, and they did so via an analysis of more than 1.2 million births whose details were recorded on the Swedish Medical Birth Register.
And what they found was fascinating.
The women who had the highest chance of a large discrepancy were those who had a body mass index of 40 kg/m2 or higher.
They aren’t the only ones. Kullinger et al (2016) found that younger women, women with diabetes or gestational diabetes, women having their second or later baby and a few other groups were also affected. There’s more detail on this in my books, but the detail is really secondary to the fact that, because there is so much variation overall, due dates aren’t very reliable generally.
Overall, Kullinger et al (2016) found that more than one in six women had a discrepancy of more than a week.
Which is a real problem when they are being used as the yardstick to decide when women are offered induction, an intervention with medium- and long-term risks, which often leads to other interventions and means that a woman is more likely to end up having a caesarean.
Back to the issues for women with a higher BMI
As I discuss in my Plus Size Pregnancy book:
“It’s important to know that a woman with a higher BMI is more likely to have a longer pregnancy than someone with a lower BMI (Denison et al 2018, Slack et al 2019).
This can mean that more women with a higher BMI are advised to have induction for post-dates pregnancy relative to those with a lower BMI.
But both pregnancy length and BMI have a genetic element, so longer pregnancy may simply be a familial tendency and ‘normal for them’.
It’s difficult to be sure about this, partly because women with a higher BMI are also more likely than women with a lower BMI to be given an inaccurate due date if they have a scan to date their pregnancy (Källén et al 2013, Kullinger et al 2016).
This is particularly ironic and frustrating as we know that larger women have more scans.” (Wickham 2023).
What can we do?
Our culture has become very attached to due dates, despite their fallibility on a number of levels.
It will take a lot of work to turn that tide.
But there’s plenty of room for discussion.
One starting point is to ensure that individual women and families are well informed. That’s especially the case for those who are most likely to be affected.
If you’d like to know more, I have written several books that look at these issues in depth and discuss the evidence in a clear, friendly way.
References
Kullinger M, Wesström J, Kieler H et al (2016) Maternal and fetal characteristics affect discrepancies between pregnancy-dating methods: a population-based cross-sectional register study. Acta Obstetricia et Gynecologica Scandinavica.
Wickham S (2023). Plus Size Pregnancy: what the evidence really says about higher BMI and birth. Avebury: Birthmoon Creations.
About the Author: Dr Sara Wickham is an author, speaker, and researcher specialising in pregnancy, birth and maternity care. Her work focuses on evidence-based, woman-centred information and informed decision-making, drawing on more than 30 years of midwifery knowledge and experience.
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