Estimating Due Dates

The day you will probably meet your baby!! Or not. Did you know that only 4% of babies arrive on their due date? This is because it turns out we’re not as good as predicting babies’ birthdays as we thought we were.

There are several ways of estimating when your baby might arrive and some are more reliable than others. Why is this important? Well, if you ‘go over’ (your baby doesn’t arrive in or around his due date), having the most accurate due date possible can help you to avoid the dreaded induction (more on this in a later post).

Some of the ways your health care provider has to estimate your baby’s expected due date are:

-Based on the date of your last menstrual period (LMP)
-A Combination of Ultrasound and LMP


pregnancy calendarBased on the date of your last menstrual period

When you first visit to your care provider to confirm your pregnancy this is probably how your GP or practice nurse will estimate your due date. They’ll pull out their little due date wheel, which usually is based on Naegele’s rule. There are a couple of problems with Naegele’s rule though.

  •  Firstly, Naegele wasn’t 100% clear whether he was calculating from the first day of the last menstrual period or the last so health care practitioners can differ on this point. Depending on the length of your period, which varies from somewhere between 2 and 8 days between women, this alone means your estimated due date could be out by 5 days or even more.
  • Naegele’s rule also assumes that you have a 28 day menstrual cycle which we know is not the case for lots of women, as our menstrual cycles can vary between 23 and 36 days in length and for some women the cycle can even vary in length from one cycle to the next.
  • Naegele’s rule also assumes that all women ovulate on day 14 of their menstrual cycle. Again we know that this is not the case. Ovulation usually happens about 2 weeks before the next expected period, so if your cycle is 36 days for example, you may not ovulate until day 22. That’s over a week after Naegele expected you to ovulate!

Something else that appears in the research with regard to LMP  is that only about 56% of women actually recall exactly the date of their last period and that women tended to under-report the length of time since their LMP rather than over-report. So chances are you may be overestimating how many weeks pregnant you are if you’re just going on your LMP.

Ultrasound due dates


Most mums are offered an early ultrasound when they’re booking with the hospital or obstetrician who will be providing their care. In 2008 NICE (the National Institute for Health & Clinical Excellence) recommended that ‘all pregnant women should be offered an early ultrasound scan to determine gestational age’ With regard to ultrasound for estimating a baby’s due date, the research suggests that:

1. An early ultrasound (most accurate at between 11 and 14 weeks of pregnancy) is more accurate than LMP.

2. Accuracy of ultrasound significantly declined from about 20 weekso of pregnancy onwards and because of this decreased accuracy, ACOG (the American College of Obstetrics and Gynaecology) recommends that ‘due dates should only be changed in the third trimester in very rare circumstances’ for example,

– if it is the woman’s first ultrasound

– if it is more than 21 days different than the E.D.D

Also important to note is that NICE’s 2008 recommendations state that:  ‘Induction of labour should not be carried out simply because a healthcare professional suspects a baby is large for gestational age’, (ie induction for a ‘big baby’).

Combination of LMP and UltrasoundHappy parents with children waiting for newbaby, pregnancy

A large study in 1997 showed that the use of an ultrasound scan combined with an accurate date of LMP showed a reduction of 70% in the number of pregnancies considered post term (or ‘overdue’). ACOG and NICE both recommend looking at both the ultrasound and date of LMP to determine what your due date might be.

So if your care provider estimates your due date based on both your ultrasound scan and the date of your LMP (if you’re pretty sure), you’re much less likely to find yourself past your E.D.D and wondering when baby is going to make an appearance!

If you do find yourself in this situation, however, don’t worry. A recent small study found that ‘the length of human pregnancy can vary naturally by as much 5 weeks’ and NICE advises that pregnant women should be aware ‘that most women will go into labour spontaneously by 42 weeks”. Maybe some day, we’ll be asking about a due week or a due fortnight or even a due month!

If you have more questions about evidence-based care and are ready to start preparing for your positive birth, get yourself along to a Gentlebirth workshop!