The time frame we have been given by KEMH is 11 weeks to 13 weeks and 6 days. However, if your baby is measuring slightly smaller than gestational age, they may have difficulty getting an accurate measurement. That might be why they are encouraging you to wait a while longer. If you are sure of your dates just tell them that you are booked to go on holiday and wont be back until after 14 weeks gestation, that way they really have no choice and its only a little white lie.

Forgot to say, Max only measured slightly over 11 weeks when we had the NT scan, the girl was hesitant at first but decided to give it a go.