I'm going to fire a pregnant employee
Are you being serious? Why would you fire a pregnant employee?
Her pregnancy is getting in the way of her work.
While it is okay to fire people most of the time, firing a pregnant employee will hurt your reputation. People will think you are heartless.
So what? If she can't do the job then she shouldn't be here.
I think you should give her a bit of a break. It's very difficult to carry a child, and I'm sure she's doing her best. I think it would be bad for the morale of the office if you fired her.