Why should i get a chronicity scan?

If carrying twins: Twins may or may not be at risk of the blood vessels from which they get nutrition and oxygen from the mother being shared. This sort of scan is intended to help determine that. If there is such sharing then there is risk of a "twin-twin transfusion" that can cause severe problems, usually for the baby that receives too much blood.

Eval Twin pregnancy: I imagine you're carrying twins? The only way to be sure the twins are not identical is to show they are not the same sex. If twins are identical there can be connections between the placentas that allow more blood to flow to one twin than the other resulting in a small twin with too little fluid and a large twin with too much fluid and possible heart failure (twin to twin transfusion syndrome).