2. Your doctor would refer you for ultrasound screening as part of a routine check-up at this stage in your pregnancy. This is used to see that the foetus is growing normally, including limbs, heart, brain and internal organs. It can also help doctors detect some abnormalities earlier than would be otherwise possible.he ultrasound can be carried out before or after this time, but it is recommended to have the scan from 17 weeks onwards and preferably before 21 weeks, as this is the best time to see all the structures clearly.