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.
What state do you live in? I work in TX at a hospital cleaning , same here lots of lifting, pushing, pulling etc.
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