Originally Posted by Tempest

They went into the tent when they weren't supposed to and were warned to stay out. Can't remember for sure but I think that was why she ended up losing her son. i.e. the demons involved with saving Drogo "took" her son as well.

hmmmm... There could be more to it though given it wasn't normal.. She's "special" being one of Dragons.. Being like that, it could also be that she can't breed unless it's with one of her kind.. There was an offhand comment at one point I think where the Queen had told Ned that the old royalty (the line of the Dragons, Daenerys' ancestors) had bred within their own family to keep the blood lines strong. Could actually have been more than that.