You'll be given a report of the scan which will tell you exactly how many weeks pregnant you are.
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.
How big your baby will be depends on when you have the scan: . Consent standards and guidance for Down's syndrome and Fetal Anomaly screening.
Most units offer a screening test for Down's syndrome at the same time as the dating scan.
If you do have a screening test, you'll be offered the dating scan first. National Collaborating Centre for Women's and Children's Health.