When discussing your work and pregnancy with your manager in future put it in an email so it's all documented.
DON'T do the extra work and don't work overtime anymore - do your hours then go home. If they fire you well, that won't look good for them at all if you dispute it. I'd sooner get unfairly fired than quit if this was happening to me.
Look after yourself. Get up and go for walks around the office regularly as your pregnancy progresses, very important.