One of the things that concern many mothers expecting a baby is the ‘due date’as women want to know when they will meet their bundle of joy! Carrying a baby for 9 months is a momentous task and it’s easy to get fixated on the expected date of delivery given to you by your doctor. Disappointment can set in when a due date comes and goes with no sign of labour. Or perhaps what a woman thinks is early labour signs (show, braxton hicks, pelvic pressure) has had her convinced for weeks that the baby will come early and again, nothing happens. Instead she is preparing for her 41 week doctor or midwife appointment.

So what exactly is a ‘due date’ and should your baby arrive when expected?

German obstetrician Franz Karl Naegele (1778-1851) suggested taking the first day of the expectant mother’s last period, adding one year, subtracting three months, and adding seven days. This can vary by up to three days from the usual system used today – adding 280 days (or 40 weeks) to the first day of the last period. In France it is standard to add two weeks and nine months to the first day of the last period – or a total of 41 weeks .

Data from the Perinatal Institute, a non-profit organisation, shows that an estimated date of delivery is rarely accurate – in fact, a baby is born on its predicted due date just 4% of the time. Thats only 4 in 100 babies actually born on the expected date! Most pregnancies last anywhere between 37-42 weeks gestation.

According to the UK office for national statistics, in 2016 the number of  live births occurred at the following gestations in England and Wales:

Number of weeks                             Number of births

37 – 38 weeks 53,342
38 -39 weeks 96,943
39 – 40 weeks 175,522
40- 41 weeks 180,804
41- 42 weeks 114,984
42 weeks & over 17,726


*This includes all births including Caesarean sections and  inductions of labour.

From this you can see that the majority of women had their babies after their due date. More women went over due and gave birth between 41-42 weeks than between 38-39 weeks. It could be suggested that the majority of women have their babies between 39- 42 weeks even though a due date of 40 weeks is given.

So what can affect when you will have your baby?

The due date is based on the assumption that you will ovulate in the middle of a 28-30 menstrual cycle. Many women wont ovulate on day 14 or 15 and many women have longer cycles. This is why ultrasound is often used to give a more accurate gestation and due date. Even with technology however, the true gestation can be inaccurate by a few days.

Ethnicity can have an impact on when you will give birth. Many women from south Asian countries will naturally go into labour and give birth prior to 40 weeks and many women of East African origin carry their babes to 42 weeks and longer.

Some women will look at their own mothers and when they had their children. If your mother had 5 children and went 2 weeks over due with them all, it might be likely that the same will happen with you.

The best way to look at a due date is as an estimate. As you only have a 4% chance of baby arriving on the given date, try not to be too focused on it as baby will come when thy are ready!