I thought you don't have to worry about it until you start showing, but I told my instructor anyway and she gave me two articles from yoga magazines. One said no yoga in the first trimester. The other said no twisting, holding breath, jumping, or inversions in the first tri and no yoga at all in weeks 11-13. Neither gave a reason why. It could be one of those yoga-y zen things. My instructor said she felt it was a little over the top, but that it's my decision. The articles might be online. I'll look when I get home.
I don't remember all the later preg stuff, but it was more of what you'd expect: no laying on the stomach starting in the 4th month, don't lay flat on your back because it can cut off blood flow, modify to go around belly, etc.
I'm going to keep going, but am trying to be more gentle.