I have had 1 at 8 weeks (dating scan), NT at 12 weeks, morphology at 20 weeks and I also had a little peeky one at my OB at about 16 weeks, just to have a look!!! I think I will also have one at about 32-34 weeks...
PK - great news you have areferral, I am sure a glipmse of the little one will get you feeling good! enjoy!