Date for a nuchal

[Ruth Walker]

Well AF is nearly 2 weeks late so I've done a test and got a BFP. With Sam I didn't test till 13 weeks pregnant so missed the opportunity for a nuchal. Don't want to miss out this time so can someone tell me, is the optimum time to have a nuchal 12 weeks from the first day of your last period or 12 weeks from the date of conception (approx 2 weeks later)? Obviously I realise its early days and I might not get that far...