Miscarried for the second time.

My doctor can't find anything wrong with me. She says I have just been really unlucky and I have just as good a chance as keeping it next time. I have a left tilted uterus but she says that shouldn't affect anything ever and my husband smokes less than a pack a day. She says that shouldn't influence it either. Anybody have any ideas? My progesterone is normal and I show signs of a thyroid disorder but blood work always comes back negative.