After scan

I had my 12 week scan last week for my twins for the Down syndrome and all that the ultrasound guy told me fluid behind both babies is perfect, I do know they need to match my bloods with the fluid test the doctor hasn’t called me so I called them and she has asked for a follow up non urgent. Should I be worried if the ultrasound was ok and I’m now referred to the hospital why would I need a follow up? Anyone had a follow up and everything was ok?

Thanks