This is because blood tests for Down syndrome are not very accurate if you are having more than one baby.
However, some major problems can be seen at your dating scan, such as problems with your abdominal wall or with your baby's skull.
You may need a second scan to check the first scan's findings.
Looking for abnormalities isn't the purpose of this scan, though.
That's the job of your next routine scan at about 20 weeks, when more detail can be seen as your baby grows bigger.
For most mums-to-be, the first scan will be a dating scan at between 10 weeks and 13 weeks plus six days of pregnancy.