look up articles by melanie dreher, RN, PhD, FAAN. she conducted some very interesting studies on marijuana and pregnancy in jamaica. you can find more information
here. some midwives recommend it for nausea! not to say that i'm necessarily advocating it - just that many women do it with no ill effects to themselves or the babies. there's an article somewhere on mothering about a woman who used it for hyperemesis gravidum (super super super bad nausea during pregnancy) successfully.
but as with anything you put in your body during pregnancy, it's basically a risk/benefit analysis, because the fewer drugs you have in your system, the better it is for your baby, since baby's getting them through the placenta. the less you expose them to during this fragile time, the better. but if your options are to be super stressed out, or sick all the time, or whatever else it can help, that's not great for the baby either. based on a lot of research studies by a lot of universities since the 1930s, they haven't REALLY found anything really bad that happens. of course, it's always wise to have a "better safe than sorry" approach if you don't feel you truly need it. but barfing all day long, losing weight from nausea/inability to keep food down, or constant stress/anxiety have proven ill effects to your baby, whereas weed doesn't. so... yeah.