You probably know when your LMP was, but not when you actually conceived.
If you do know the conception date, this is counted as day 14, and not day one.
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.
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.However, the timing of your first scan will also depend on how your pregnancy is going, and where you live.Your midwife may recommend an early scan at between six weeks and 10 weeks if you have experienced bleeding.You may also be offered an early scan if you've had a previous miscarriage.