What is a ‘due’ date and how is a due date calculated?
Your “due date”, in the context of pregnancy, is an estimated date for when your baby is likely to arrive. This date is typically provided during your first appointment with your midwife. Most healthcare professionals calculate it using Naegele’s Rule, which assumes you have a 28-day menstrual cycle and ovulate on the 14th day. However, every woman’s body is different; some women have shorter or longer cycles and may not ovulate on day 14. While some providers may adjust the due date based on the length of your cycle, many still rely on Naegele’s Rule to estimate your baby’s arrival during those early appointments.
Why is Naegele’s Rule used?
Over 200 years ago, in 1744, a Dutch professor named Hermann Boerhaave analysed the records of 100 pregnant women, to explain how to provide an estimated due date. From this data, obtained from a relatively small group of women, Boerhaave concluded that the estimated due date could be calculated by adding 7 days to the last period, and then adding nine months (Baskett & Nagele, 2000). However, the major issue with his method is that Boerhaave never clarified whether the 7 days should be added to the first or the last day of the menstrual period.
Later, in 1812, a German professor called Carl Naegele drew on the research from Professor Boerhaave and added some of his own thoughts. It is at this point that ‘Naegele’s Rule’ got its name. Again, like Boerhaave, Naegele did not say when you should start counting; his notes can be interpreted one of two ways: either you add 7 days to the first day of the last period, or you add 7 days to the last day of the last period. This was confusing for medical professionals who needed clarity.
As time went by, doctors construed Naegele’s Rule in different ways which led to inconsistencies between professionals. Initially, more doctors chose to add 7 days to the last day of the last period. Despite this, by the 19th and 20th centuries, adding 7 days to the first day of the last period and then adding 9 months started to become common practice in the UK and other countries, as medical practices and textbooks adopted it.
Today, doctors and midwives in the UK still rely on a version of Naegele’s rule that adds 7 days to the first day of your last menstrual period and then counts forward, assuming a 40-week pregnancy. It’s surprising that this method remains standard practice despite lacking current evidence and possibly not reflecting what Naegele originally intended when he developed the rule. Even more astonishing is the lack of global consistency in calculating due dates — for example, in France, pregnancies are estimated to last 41 weeks, meaning your due date there would typically be a week later than in the UK.
Although Naegele’s rule is commonly used during your early antenatal appointments, your sonographer will also estimate your baby’s gestational age during your “dating” ultrasound, usually scheduled between 10 and 14 weeks of pregnancy. This scan measures the size of the embryo, and sometimes your due date may be adjusted based on these findings. While the dating scan is generally seen as more accurate than earlier calculations, it’s still just an estimate and can’t predict the exact day your baby will arrive.
Will my baby arrive on its ‘due’ date?
We often put a lot of weight on our due date once it’s given, but is that really necessary? It’s important to note that only about 4% of babies actually arrive on their exact due date. For many expectant mums, the pressure of “D-day” can cause stress, especially when medical interventions like inductions start to be discussed. The World Health Organisation (WHO) defines a normal full-term pregnancy as anywhere between 37 and 42 weeks — so should we really worry if our baby hasn’t made an appearance by their due date?
Research helps to shed some light on this. A 2001 study found that half of first-time mums gave birth by 40 weeks and 5 days, and 75% by 41 weeks and 2 days. For mums who had given birth before, half delivered by 40 weeks and 3 days, and 75% by 41 weeks. This means pregnancies often last a bit longer than the traditional due date calculated by Naegele’s rule — around 5 days longer for first-time mums and 3 days for those who’ve given birth before. Another study showed that the median pregnancy length is about 40 weeks and 5 days.
So, rather than stressing over the due date as a strict deadline, maybe it’s more helpful to embrace the mindset that “my baby will come when my baby is ready.” After all, the due date isn’t an eviction notice — it’s just an estimate.
- Baskett, T. F. and Nagele, F. (2000). Naegele’s rule: a reappraisal. BJOG 107(11): 1433-1435.
