Once you have confirmed that you are pregnant, you will want to find out when your baby is due to arrive. The average pregnancy is calculated at 280 days (40 weeks) from the first day of your last menstrual period.

However babies rarely keep to an exact timetable, so a full-term pregnancy can be anywhere between 37 and 42 weeks. A baby born before 37 weeks is considered to be premature and anything past 42 weeks is considered overdue.

In order to calculate your baby’s due date, add seven days to the date of your last period and then add nine months.

For example, if your last period started on 1 March, adding seven days will make that 8 March. Then adding nine months will give a due date of 8 December. If your periods are irregular or you are unsure of the date, an ultrasound will help determine the development of the embryo and your due date. Ultrasound scans can be done at any stage of pregnancy after the first six weeks.

You can use the Pregnancy, Birth and Baby due date calculator to work out when your baby is due.

Due dates are usually calculated on your last period instead of the date of conception because of a number of reasons.

• Although the average woman ovulates (releases an egg) approximately 2 weeks after her period, the exact time is not always known.
• Once an egg has been released, it can remain fertile for up to 24 hours.
• Sperm can last for up to 7 days after intercourse to fertilise an egg.

Some doctors will refer to your due date as ‘expected date of confinement’ or EDC.

Sources: Better Health Channel (Baby due date) , John Hopkins Medicine (Calculating a due date) , Mayo Clinic(Pregnancy week by week)