Ultrasound scans

A scanner probe is placed on your skin and very high frequency sound waves that are inaudible to the human ear are passed into your body. As the sound waves pass over objects in fluid, they produce a pattern of echoes . These echoes are converted into electrical signals, which are processed and displayed on a screen as a two-dimensional image.

As the scanner probe is guided over your abdomen, you will feel only the cold plastic. If the pregnancy is at a very early stage, you may need to have a full bladder for the scan to be effective; this may cause slight discomfort. You shouldn't feel forced into having a scan. Scans have been offered as routine for about 15 years, but if you feel that this is unnecessary interference and you don't need extra reassurance, you can decline

When you will have the scan depends on your hospital, but an early scan may be offered 8 to 12 weeks after your last period, so as to date the pregnancy. You will be given a routine scan at about 18 to 22 weeks to check the development of your baby's organs. There are several reasons why a scan is offered:

• to measure the baby and so give an accurate delivery date
• to find out how many babies you are carrying
• to check for any complications by looking at the baby's limbs, organs, brain, and spine
• to check that the placenta is not lying over your cervix
• to see the position of the baby.

Further scans may be recommended if it was not possible to see everything clearly because of the position of the baby. More detailed scans that investigate particular problems, such as slow growth or chromosomal abnormalities, may be required. You may also be offered a further scan within the last six weeks of your pregnancy to check on the position of the placenta, or to establish the baby's position.