Dating ultrasound too early
The dating scan will also show if you're expecting twins, triplets or more.
However, the timing of your first scan will also depend on how your pregnancy is going, and where you live.
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.
This is because blood tests for Down syndrome are not very accurate if you are having more than one baby.
Instead, you will be offered a nuchal translucency (NT) scan.