Historically, employers frequently exclude females, or at least pregnant females, from certain jobs based on the potential harm that could result to the women or fetuses within them from certain conditions of the job.
Examples: Jobs involving exposures to toxic chemicals or jobs involving heavy lifting.
The Johnson Controls case, decided by the U.S. Supreme Court in 1991, appears to have finally invalidated such fetal-protection policies as gender discrimination, unless the policies are equally applied to males. More specifically, the case held that decisions as to whether females who are pregnant or might become pregnant should work at potentially hazardous jobs, assuming they were physically able to perform the duties of the jobs, should be left to the females themselves rather than to their employers.