The effects of body vibration are talked about in this page.
H TeeTeePee://homepages.cambrianc.on.ca/johs/pages/BodyVib1.htm

According to OSHA 1910.178(l) - Questions and Answers (Appendix A)...
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#2)
Can an employee with poor vision in one eye or a hearing impairment be allowed to operate a powered industrial truck?

The employer has the responsibility under this training standard to ensure that each operator is capable of performing the duties that are required of the job.

The Americans with Disabilities Act (ADA) addresses the issue of whether employers may impose physical qualifications upon employees or applicants for employment. The ADA permits employers to adopt medical qualification requirements necessary to ensure that an individual does not pose a "direct threat to the health or safety of other individuals in the workplace," provided all reasonable efforts are made to accommodate otherwise qualified individuals. The employer should consult with appropriate medical personnel to assist in determining operator physical qualifications.
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In addition, the General Duty Clause suggests that if there is physical harm to the pregnant operator (or baby), then you would have support to remove the operator from the forklift:
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"Section 5(a)(1) of the Occupational Safety and Health Act requires an employer to furnish to its employees:
employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees...."

This could be a touchy subject. Have not heard of this issue coming up but I haven't heard it all yet. But I might suggest that a monthly or more often letter from her OBGYN doctor (s) saying it is OK would be a starting point.