You'll be given a report of the scan which will tell you exactly how many weeks pregnant you are.
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.
After 13 weeks your baby can curl up and stretch out, so measuring the length becomes less accurate.
Then the circumference of the head becomes the best way to measure your baby Some hospitals and private clinics offer a dating and screening scan at the same time.
Your midwife may recommend an early scan at between six weeks and 10 weeks if you have experienced bleeding.