My question involves labor and employment law for the state of: MO

From the very begining of my employment my employer knew I had Rheumatoid arthritis. The job is in pharmacy and everyone stands on their feet for all 8 hours a day, although not all tasks require standing. Lately my arthritis has gotten worse and now I am pregnant so things have gotten difficult to do while on my feet. I have suggested that I bring a chair to work and take the responsibilities that can be done sitting down. They refused saying that it wouldn't be fair to everybody. Now they are asking that I cut my hours and demote myself after I told them I will not be coming back to work full-time after maternity leave. They even have asked that I try to get a letter of disability so I can step down and let someone else take my place. Can they do that if I don't want to step down before maternity leave?