If you’re pregnant, how accurate is your ‘due date’?

More than 1,700 pregnant women in South Australia have reportedly been given the wrong “due date” due to a technical error in their computerised medical records. This has prompted concerns some women may have had an early induction of labour as a result.

According to today’s ABC report, most of the women whose records are affected have since given birth, while about 100 are still pregnant. Not all women have yet been notified of the error.

An investigation is now under way to find out how the error happened and if the incorrect due dates have impacted care.

But how accurate are due dates anyway? And what happens when we get them wrong?

What is your due date?

What people normally call their “due date” is actually the estimated day your baby will be born. There are a variety of online tools to work out this date, based on something called Naegele’s rule, named after the German obstetrician Franz Naegele (1778-1851).

Naegele’s rule works out your estimated due date by taking the first day of your last menstrual period, adding seven days, then adding 40 weeks (nine months).

Naegele’s rule has limitations. First, it assumes all menstrual cycles are 28 days long and ovulation occurs on day 14. It also relies on the woman knowing the date of her last menstrual period.

This method is still used by doctors and midwives to work out the expected date of birth.

However, there is an increased reliance on using an ultrasound between eight and 16 weeks to predict the due date. This measures the size and development of the fetus to calculate its gestational age. From there, the sonographer uses that data to work out when the baby is due.

There’s also a trend towards identifying a date range rather than an exact due date.

A large retrospective study of over a million births in Germany found 90% of births occurred between 19 days before to seven days after the expected due date. That is a range of 37 weeks to 41 weeks’ gestation.

Why are due dates important?

Your due date is important for a number of reasons, beyond allowing you and your family to plan for the months ahead. It’s a key piece of information your midwife or doctor uses to monitor your pregnancy, assess whether everything is on track, and if intervention is recommended.

One intervention closely linked to your expected due date is the offer of induction of labour (being induced) if you are “overdue”. For women who have had their estimated due date incorrectly calculated there is the possibility they will be induced earlier than required.

Using an ultrasound-predicted date can give more accurate dating than using Naegele’s rule, perhaps avoiding unnecessary induction of labour if the woman is not really overdue. However, more evidence is required to confirm this.