It's entirely up to you whether or not you have a scan. You could have a dating scan without NT screening, or even change your mind on the day of your scan. The point of the scan is to work out how many weeks pregnant you are. Ectopic pregnancy and miscarriage: diagnosis and initial management CG 154.
Your baby's head, limbs, hands, and feet can be seen, as well as his abdominal wall and some organs, such as his stomach.
The dating scan isn't designed to look for more subtle abnormalities, though.
It may also help you to avoid feeling disappointed towards the end of your pregnancy if you think your baby is late when he's not.
The scan can check that your baby's heart is beating nicely and that he's developing normally.
As long as the timings are right, you can have the NT scan and blood test at the same appointment as your dating scan. That's because the screening test needs an accurate due date to be reliable.