Best time for NT scan?

[rosalux]

I am aware that the NT scan needs to take place between weeks 11 and 14, but I have also read conflicting advice as to when in that period is the ideal time. I have seen both 12 and 13 weeks suggested and am unclear as to the implications of testing earlier or later as regards accuracy etc. Any ideas?

FWIW I booked mine recently and the hospital (Rosie in Cambridge, which has a good reputation) were determined it had to be AFTER I'd hit 12 weeks (that'll be next Fri, the 21st and my scan is on Mon 24th, so I guess that's week 13?), even though I was lobbying for it to be done before next Fri so I would be able to tell people at a family do on the 22nd. Frivolous reasoning alert. Anyway, they were quite firm, and it wasn't due to lack of availability. But no idea what the implications are. They know pretty exactly how far on I am, as I had an earlier scan, so I guess they've decreed the optimum time?