Pregnant and Pressured: How to Handle Forced Resignation and Unfair PIP at Work?

I joined my company on 4th June 2018, and I conceived in July 2018. I informed my company verbally about this to change my process, as I am in the sales department and under a lot of pressure. Now, my company is forcing me to resign and obtained my signature forcefully on a PIP (Performance Improvement Plan) and increased my monthly target, leading to a lot of tension to save my job. They have denied changing my process as well.

What should I do in this condition?

Can they fire or terminate me based on my performance? They are mentally torturing me. I cannot survive without a job. Please advise on the solution to fight with them.

"Please advise the solution to fight with them."
Fight will get you nowhere. You need to convince them, as the member above has written.
What industry are you working in?
The company knew about the pregnancy when you joined, and still, they are giving you higher targets.
Try to keep options open for another job as you are in a tough situation with a small child and tough targets.