An ultrasound scan can be used in several ways:

• To check your baby's measurements. This gives a better idea of how many weeks pregnant you are. This can be useful if you're unsure about the date of your last period or if your menstrual cycle is long, short or irregular. Your due date is likely to be adjusted according to the ultrasound measurements.
• To check whether you're carrying more than one baby.
• To detect some abnormalities, particularly in your baby's head or spine.
• To show the position of your baby and the placenta. For example, when the placenta is low down in late pregnancy, special care may be needed at delivery, or a caesarean section may be advised.
• To check that the baby is growing and developing normally (this is particularly important if you're carrying twins or more).

At the scan
You'll probably be asked to drink a lot of fluid before you have the scan. A full bladder pushes your womb up and this gives a better picture. You then lie on your back and some jelly is put on your abdomen. An instrument is passed backwards and forwards over your skin, and high-frequency sound is beamed through your abdomen into the womb. The sound is reflected back and creates a picture, which is shown on a TV screen. It can be very exciting to see a picture of your own baby moving about.

Ask for the picture to be explained to you if the image seems confusing. It should be possible for your partner to come with you and see the scan. Many couples feel that this helps to make the baby seem real for them both. Ask if it's possible to have a copy of the picture. There might be a small charge for this.