I got fired today, and I'm pregnant. Can they do that?

I know I have not been the model employee since getting pregnant. I have had alot of morning sickness, so I have missed work. I work in a call center, and they keep record of your stats, personals, breaks, meetings ect. My stats were higher than normal because of having to use the washroom alot. I had my doctor fill out a form to excuse me from the attendance policy, because of my being sick. My employer then made me part time, then reduced my hours to the minimum for the company. I had given a doctors note exactly 7 days ago saying I was able to work. But since then missed three days due to morning sickness. I'm 29 weeks pregnant, and because of getting fired, I cannot make my maternity leave. Can they do that?