My job on the ambulance doesn't offer light duty either. I was told prenancy is not an injury /disability. Its a shame because it is hard work and lots of lifting
What state do you live in? I work in TX at a hospital cleaning , same here lots of lifting, pushing, pulling etc.
Did your doctor asked you to do light duty because honestly although pregnancy does get you tired I don't think its a reason to stop doing activity.. however if you feel physically drained I'm sure your doctor can give you paperwork which you can present to your HR department.