You may have calculated the length of your pregnancy from the first day of your last menstrual period (LMP).
This is even though you don't actually conceive until 14 days after your LMP, or later than this if your cycle is longer than 28 days.
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.
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.
Keep it with your maternity notes so your midwife and anyone else involved in your care can see it. This measurement, known as the crown rump length (CRL), is very accurate in the first trimester.
Finding out about having twins early in pregnancy also gives you more time to prepare for the birth and for your doctor or midwife to plan your care.
Knowing about twins early is also useful if you want to have screening for Down syndrome.
If you are having a screening test for Down syndrome, you'll be offered a dating scan first.
The test needs an accurate due date to be reliable, as hormones in your blood vary according to your stage of pregnancy.