On a purely anecdotal level, I know quite a few people who smoked pot while pregnant.
In the case
of one
of my best friends, her kids had relatively low birth weights
in the 5-6 pound range but she's also extremely tiny to begin with. I haven't observed any negative effects in the children, but then none of them are very old yet either.
I agree that smoking anything, regardless of chemical content, is going to take oxygen away from your developing child though.