Doctors and midwives calculate due dates by taking the first day of your last period as day one and the due date as 40 weeks from then. This is most accurate for women with a regular 28-day menstrual cycle as it is based on 14 days to ovulation and fertilisation occurring approximately then. First time mums might be more likely to go overdue.
Statistically speaking, there’s only about a 4-5% chance your baby will be born exactly on their due date. Around 80% of women deliver somewhere between 38-42 weeks. This is called ‘at the term’ so the baby is considered to be fully developed. Around 11% of mums give birth to their baby prematurely i.e. before 37 completed weeks. Sometimes they will spontaneously go into labour and sometimes the decision will be to deliver the baby immediately, either by caesarian or by artificial induction of the birth. In addition, multiple pregnancies tend to come early. Underweight women have a higher chance of delivering early as well.
Births after 42 weeks are called ‘post-term.’ Around 5-10% of pregnancies last longer than 42 weeks, and at that point, hospitals usually induce labour as the rates of stillbirth increase after this time. Induced births carry their own risks and are often more difficult for mums to cope with as the contractions can be very strong and cramping – but the pros of induction are largely thought to exceed the pros of letting the pregnancy take its natural course.
After your dating scan (see our article on scans) your due date may be updated, since it often gives a more accurate reading.
Our nutrition expert, Melissa Little, talks through some of the ideas people have about inducing labour.