I would say that any time spent in a chemistry lab would be a big risk for a pregnant woman. As good as fume hoods are, there is also the concern of chemical absorption through the skin as well as the possibility of a chemical spill outside of a fume hood. Furthermore, in my experience as a lab TA, rarely do students keep their fume hood sash low enough to comply with OSHA standards.
Think about the things that you aren't supposed to do while pregnant: handle cat litter, eat artificial sweeteners, drink alcohol, expose yourself to second-hand smoke, etc. Think of it this way: everything you are exposed is magnified in your baby. I worry about my exposure to chemicals everyday, to the point that I think it may be dangerous to me, so it can't be any better for a pregnant woman.