Work-pregnancy question

I am currently a CNA. Long hours on my feet, constant walking, a lot of bending over, and pushing/pulling/lifting. I have amazing coworkers so even as of now they don't let me push pull or lift someone who is not easy to do. But when I'm much more pregnant and waddling and can no longer get up the halls fast enough to answer call lights, when can I ask my doctor to put me on light duty? 7 months? 8?

Have any of you had similar work experience and being pregnant?