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Dating scan

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In some areas of the country, you will have a 'dating scan' at around 12 weeks. Not only will this confirm your pregnancy, but it will also give the sonographer a chance to check the size of your baby and see if it corresponds with the due date you have been given, based on your late menstrual period (LMP).

There are charts that show the average size of foetuses at each stage of gestation, and if the size of your baby is bigger or smaller than normal for the stage you think you're at, your due date may be adjusted.

This doesn't mean your baby is abnormal or isn't growing properly, just that your original due date may have been a bit early or late. This could be because your menstrual cycle is longer or shorter than average,or that you ovulated a little earlier or later in your cycle.

If you've had IVF or other fertility treatment, you'll be given an early scan to confirm the pregnancy and predict a due date.

Why it can be wrong

This method is more accurate than the one worked out according to your LMP but, again, it's based on averages, in this case the average size of unborn babies at each stage. And just as some little ones are bigger than others at birth, some are naturally bigger or smaller than others at 12 weeks' gestation.

The other problem is that not all women are offered dating scans, many don't have a scan until close to 18 weeks. Scans up to 12 weeks' gestation accurately predict an EDD, but after 13 weeks they're only accurate to within about two weeks.