I have some knowedge in this area due to working in the petrochemical industry for 12 years and having a couple of timekeepers and other office staff having pregnancies.
And it can get really sticky here. What little OSHA says on the subject is confusing, however EEOC is very clear. My suggestion to you is to determine the chemicals the pregnant employee may be exposed to and which ones can harm a fetus. Sit the employee down and go over worst case senarios explained as such. Then do your best to accomodate her into a temp position outside the exposure areas. do this is a way that your company can deal with and something that does not upset her. Also, make sure she is able to have her other job back after she has the baby shiould she want it back.
All that said, if you cannot work out any accomodations and she chooses to stay in the job she has you cannot move her without a potential EEOC lawsuit. Here's the craviest thing if she chooses to stay in the job and the exposure harms her baby, in most states the company will be held at least partially responsible for the harm done to her baby.
your best bet is to work something out with her.