Your baby's due date is calculated from one main factor. It's so simple that you can do it yourself with a high degree of accuracy, but a doctor will confirm their estimate when you see them about your suspected pregnancy.

The length of your pregnancy

The definition of a full-term pregnancy (i.e. not a premature, or preterm, pregnancy) changes from medical expert, to association to country. In Australia, the general consensus is that a full-term pregnancy is a baby born from week 37 onwards.

So where does 40 weeks come in?

The general rule, and the one that doctors will use, is to calculate your pregnancy for a 40-week duration. But you might be surprised to know that you aren’t actually pregnant for that full 40 weeks!


Doctors calculate your pregnancy from the first day of your last period (i.e. before you ovulated and became pregnant). Because there can be up to two weeks between the first day of your period and ovulation/conception, this means that if you did have your baby exactly on your due date, you have actually been pregnant for 38 weeks, not 40.

Calculating the date

So once you have determined the first day of your last period, you add on another 40 weeks or 280 days to that.

Is it accurate?

You can see the issue with this. The knowledge that you have your period comes only from your experience of the symptoms. Processes are occurring internally that you could have no sensory experience of. Besides that, every woman’s period is different – an egg might be fertilised on day 12 or day 15 for example – and isn’t necessarily 28 days exactly.

While menstruation is something you have probably experienced for some years now – and you understand your own cycle better than anyone – it’s still possible to have your period and the symptoms be minor or missed, and therefore you aren’t sure when the first day is.

So despite the baby’s intentions – and the best intentions of your doctor to give you a guide – it’s unlikely childbirth will occur on the date calculated. In fact, common figures for the accuracy of this estimated due date are somewhere between 4% and 10% - that is, somewhere between 1 in 10 and 1 in 20 of women actually have their babies on their “due date”.

Can the date be revised?

Yes, doctors may carry out other tests, scans or ultrasounds that can provide a higher level of accuracy by measuring other factors.

Is there something wrong if I’m early or my baby’s late?

Babies born outside their due date but within the full-term bracket are unlikely to experience health problems. Every baby is different and every woman’s pregnancy is different.

If the date’s not important, why does it matter?

Because a baby may be born full term anywhere between 37 and 42 weeks, the due date can’t be taken seriously as the due date, can it? Maybe not, but an estimated due date provides a framework that doctors and specialists can use to provide care and tests for you and the baby. They know, for example, that at week x of most pregnancies, hormone y should be detected at certain levels to indicate the absence of abnormality z.

It can also frame how you are given advice – for example, women are advised to take folate supplements until the baby’s neural tube has fully developed. Because that occurs around week 12 of a pregnancy, doctors advise women to take these for three months.

So take your due date with a pinch of a salt and use it as a guide rather than deadline – when your baby’s ready, they’re ready!

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