In my experience, the first scan at 10 weeks is to confirm a viable PG, the NT one is totally optional (ie only if you are choosing to have the test done) and then the big one at 18 weeks or so.

Having said that, there is a big deal made about measuring for dates even though the dating wasn't the motive behind the u/s. Guess it's a sign of the times that so many women either have no idea about their cycles (I was one of these first time around) or put such blind faith in what the medicos say that they aren't used to being told anything contrary to what their machines tell them...

Bit sad really!