You may find at this scan your due date is altered on your notes.

It's more common to be told you're less far along than you thought you were, rather than that you're actually further along in the pregnancy.

You should then see the black-and-white image of your baby appear on the screen.

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A low-lying placenta usually moves up later in the pregnancy but if it doesn't, it can cause problems in labour so they will want to check its movements throughout your pregnancy.

It's not completely foolproof – but the dating scan is a much more accurate way to work out how far you are into pregnancy than calculating it by the date of your last period, which is the only method the midwives will have used so far to work out your due date.

If the screening tests find that your risk for any of the abnormalities tested for is high, you will be offered diagnostic tests such as CVS (chorionic villus sampling) and amniocentesis to find out for sure if there's a problem.

You're under no obligation to have these, and they do usually carry a small risk of miscarriage as they are invasive.

I'll never forget one nurse asking me, 'Do you think you could just pop to the loo and let just a little bit out? ’ When it comes to the actual scan you'll lie on your back and then some sticky, clear gel will be rubbed over your abdomen.

The sonographer will tuck some tissues under your clothes so you don't get gel all over them.It may not be immediately obvious which parts of your baby you're looking at, but the sonographer will be able to help you work it out.He or she may need to prod your belly a bit to “encourage” the baby into a suitable position.He or she will then move a hand-held transducer over your belly, which bounces high-frequency sound waves off your baby to create a picture on the monitor.You might feel a bit of pressure but it doesn't actually hurt and won't harm your baby.For instance, if you have any abdominal scarring or are overweight the picture may not be as clear.