sara id have the NT scan done and the serum bloodtests to make sure all is ok at that 12 week mark. i guess you have to make the decision that if there is anything wrong would you be happy to continue the pregnancy, what if something major was wrong at teh 20 weeks scan, what would happen. obviously the desicion is solely yours and your partners to make.
id take it one step at a time, get these less invasive test done and go from there it cant be easy and must be very stressfull, i know your doing waht best for you and bubs