Due Date Calculation

Once the news that you are pregnant sinks in, the first thing you'll want to know is when your baby will arrive! This is known as the due date, or baby due date (abbreviated in GP notes as BDD).

A baby takes 266 days, or 38 weeks, to grow to full term. So there are around 266 days between conception and birth.

You may know exactly when and where conception took place and can easily do the calculation yourself... but most people can't pinpoint the moment they created life!

So the simplest way to calculate this very important birthday is to work out when it will be 40 weeks from the start of your last period. This is because conception is usually around two weeks after the start of your last period - but not always!

If you are pretty regular (with a 28 day cycle) you can therefore guesstimate your baby's due date by counting 280 days (40 weeks) from the day your last menstrual period started.

The accuracy of this calculation is very much dependent on how regular your periods are. If you have a shorter or longer menstrual cycle the delivery date could be earlier or later.

But don't worry. Pregnancy care websites have great online due date calculators which do all the mathematics for you! Simply put in the date of your last menstrual period and the normal length of your cycle and, hey presto, a date appears!

There are other factors which can affect this calculation. For example if you conceived soon after coming off the pill (and before a normal menstrual cycle was established) it may be difficult for your GP to give you an accurate due date. They will probably have to be guided by your baby's development and you may have to wait for an ultrasound scan or even be into your second trimester before a BDD is given.

The medical staff will use the BDD to monitor the baby's development, making sure there are no problems and baby grows at the expected rate.

It is possible that unnecessary intervention may occur if too much emphasis is put on the estimated due date with some doctors inducing labour if they believe the baby is overdue.

However most GPs are happy to leave baby well alone. Risks are unlikely to occur until after 42 weeks and most doctors will be prepared to let the pregnancy continue even beyond that if their tests show the baby, and mum, are not at risk.

At 42 weeks the placenta should be adequately supplying the baby with sufficient nutrients, but a problem could arise if the placenta fails to provide nourishment and support the baby. This is known as placental insufficiency.

Mums-to-be should not be anxious if baby does not show signs of arriving on their due date. About 95 percent of babies born from normal pregnancies arrive early or late.

Debra Aspinall is an experienced journalist, editor and leading writer for the Emma's Diary website, one of the UKs foremost pregnancy and baby websites. Debra writes on pregnancy related topics such as pregnancy & due date calculator . She also writes on women's health and beauty issues and contributes travel articles to glossy magazines in London and the Home Counties.