Doctors note

Has anyone successfully gotten a doctors note late in there pregnancy? For just general pregnancy symptoms? I would rather not start my mat leave until as late as possible. And with a doctors note I could go on short term disability with my employer. I'm a first grade teacher and am finding it really hard to be on my feet all day, going up and down stairs as well as not being able to use the washroom regularly. I'm not sure how to approach this with my doctor...