Why your estimated due date might need a rethink

If you’ve ever thought that your or someone else’s estimated due date might need a rethink, this article is for you. It’s my reflection, as a midwife and academic researcher, on what I’ve learned from researching pregnancy calculation tools.

In the olden days, before smartphones and apps, I had quite the collection of cardboard pregnancy calculators. Around the time that I started researching estimated due dates, induction of labour and the evidence relating to this (which was in about 2002), I began to seek them out with the enthusiasm of a niffler who had spotted a golden snitch.

Happily, they weren’t nearly as hard to find or catch. At one time, every other organisation would give them out to midwives, obstetricians and GPs. Conference stalls were overflowing with them and, if you mislaid yours, or got meconium on it, a colleague almost always had one they could spare. Usually tucked in the back of their paper diary, because we all had one of those back in the day as well.


The joys and tribulations of the whirly wheel

These handy calculators were designed so that, if you knew the date of a woman’s last menstrual period, you could work out her estimated due date. They often also contained handy reminders for midwives and doctors of when things should be offered, done or organised, according to the estimated due date. I have to say that the one with information about maternity benefits was so well used that it fell apart. That was truly useful information to have at my fingertips to share with women and families.

But an estimated due date, as you’ll know if you’ve read either of my books on induction of labour, is a guess with quite a wide timespan. Only five percent of babies (that’s one in twenty) are born on their due date.

These days, we have apps as well. And medical records systems, ultrasound machines and other technologies are also involved in calculating someone’s estimated due date.

But I want to share a little-known fact about pregnancy calculators, no matter whether they are the cardboard jobs that my friend Tricia Anderson and I used to call ‘whirly wheels,’ a free or paid app or – believe it or not – a computer system used by a midwife or doctor.


What’s the problem?

The fact is that they don’t all give the same date. Even when you give them the same starting point to work from.

I once tested whirly wheels by collecting about thirty of them, setting them all at the same mythical last menstrual period date and looking at the estimated due date they suggested. Later, some lovely midwife friends did the same experiment for me on their hospital record systems.

When I compared the range of dates, I found that there was a whole week’s difference between the earliest and latest date given.

I recently tried the same experiment with a series of free and paid phone apps. With the same result: even if you use the same LMP date, there is a few days variation in the estimated due date between the apps as well.


Variation is a fact of life

In Your Own Time was written to help parents and professionals better understand the issues and the evidence relating to the current induction epidemic. Looks at the evidence relating to due dates, ‘post-term’, older and larger women, suspected big babies, maternal race and more.

Does that mean that ultrasound is the answer? No. The variation exists there too.

In fact, there are many examples of how variation is a fact of life when it comes to pregnancy, birth and estimating when these might happen.

It’s not rocket science to figure out why. We’re all different. As I wrote in In Your Own Time::

“In reality, bodies, babies and pregnancy lengths vary. In reality, we exhibit individual variation, and there is actually a wide span of time in which babies can be born and be healthy. In other words, normal is a range and not just one fixed point. One size hardly ever fits all.” (Wickham 2021).

One size didn’t fit all when I started researching this area twenty years ago, and it still doesn’t now.

If you think that your due date might need a rethink, you’ve come to the right place. I’ve written more on why we might be better off using a wider window, and on who is most likely to get an inaccurate due date. More on both of those in In Your Own Time as well.


Making induction decisions

The estimated due date affects when induction of labour is offered. Induction of labour is a very personal decision. It’s absolutely right for some women and families, and not right for others.

For even more info, visit my induction resources page, where you’ll find lots of articles like this one to inform, educate and inspire.

You might also like to read one of my books on induction: Inducing Labour: making informed decisions or In Your Own Time: How western medicine controls the start of labour and why this needs to stop.

And if you’re not sure which one you need to read, I have a blog post on that here.



Want to stay up to date and get regular updates on birth-related research and thinking sent free to your inbox?

Sign up to my newsletter list.

Books to help you become informed and make the decisions that are right for you…