There was a thread about this not that long ago.....I'll have a look and see if I can find it.

I know I had my BT about 10 days before the NT scan, but I can't for the life of me remember at what point LOL! Bloody pg brain!

Will see what I can dig up for you.

ETA Okay, just found my VCGS paperwork, this is the quote from it -

"The nuchal translucency test can be done between 11 weeks 3 days and 13 weeks 6 days of pregnancy (ideally at 12 weeks). The blood test is most informative if collected during the tenth week of pregnancy, however it can be collected between 9 weeks and 12 weeks 6 days of pregnancy."

I pretty much just followed those guidlines. HTHs!