Can an employer fire someone who has a doctor's excuse for being absent?

Since my pregnancy there have been a few occasions I’ve had to call in. In the 8th week of my pregnancy I began to bleed very heavily and the ER put me on a week of bedrest. After that there have been a few days I’ve called in (almost all times I’ve had a note). My employer hasn’t mentioned any intention to fire me obviously but I just want to know my rights if that turns out to be the case. Is it legal to fire an employee for being out sick with a dr note? I’ve never been one to continuously call in. I truly hate having to do it. But there are times when health is more important than going into work to fold clothes.