Hi
I wasn't sure if your question was about taking heparin at all, or the fact he said only take it after your test is positive? As for taking it at all, it've fairly common, and there are lots of girls on it, including me.
I do know a girl who problems with recurrent m/c who was put on heparin only AFTER she fell pregnant, so I don't think it's unheard of. But for all the girls I know on it, we started after egg collection, so that it would make it easier to implant as well, as that can be a problem as Maria says. If you get pregnant OK though, maybe you don't have problems with implantation. And yes, you would be on it for quite a long time.
But honestly, if you have any doubts go and see Dr Gorgy or someone people know is good, as it's all expensive and emotional, and there's no point doing it if you're not confident you're doing the right thing.
Good luck
x