Reduced Hours During Pregnancy - Can my Employer Refuse, even with Doctor's Note?

If I get a doctor's note stating that I need reduced hours at work for the duration of my pregnancy (I'm currently 22 weeks pregnant), can my employer refuse, and if so, what are the ramifications of that?? I work 40+ hours a week, but want to work 4 days instead of 5 (so 32+ hours a week). I've already asked my employer for this WITHOUT a doctor's note, and they said NO. With a doctor's note, do they have to say yes? If not, what can they do? Will they fire me? FYI, I have been at the company over a year and qualify for FMLA. Thanks for any information you can give me.