this is from babycenter.com:
The NT scan must be done when you're between 11 and 14 weeks pregnant, because this is when the base of your baby's neck is still transparent. (The last day you can have it done is the day you turn 13 weeks and 6 days pregnant.) It's usually offered along with an evaluation for the presence or absence of nasal bone, as well as a blood test, in what's known as first-trimester combined screening.