when yo'ure pregnant a scan is to check the health of baby, it tells if it has all its limbs, if the heart is working well, and also how mature the foetus is, sometimes they can tell whether its a boy or girl. or if theres any defects.
hope this helps
First scan is at 10-12 weeks and called the dating scan; this allows the mid-wife to check the development of the foetus and the age.
The second scan is at 16 weeks and called the abnormality scan; this allows the mid-wife to check for any serious developmental problems the foetus may have.
There are other tests available but most of these have to be paid for privately e.g the Nuchal Test which can detect Chromosonal Abnormalities (Downs Syndrome).