Every one of us at some point during the pregnancy have worried about something or the other. Its normal and natural. Even though I may suggest you that try not to think about it ... still I know you would. So instead of keeping it inside ... talk it out. It may help to ease a little if not completely.
If things have been going good and u do not have any previous history of bad NT results or any family history then the chances r really low that anything will be wrong with you. Try and take comfort in this thought if u can.
Good luck .... and vent away as much u like