Is my HR manager and direct manager allowed to write me up without discussing the write up with me or having my signature?

Ever since becoming pregnant my employer has been writing me up without discussing the isses with me.Then when I became stressed at work my blood pressure became worse and my managers and HR Manager have been discussing my actions and trying to force me to quit. I've been getting write ups that ask for my signature,that i've never signed and recently I received my "final warning" without being told it was my final warning but that I only received it because I got my first customer complaint. I was recently told that it was my final and that I will be fired if I make even the smallest mistake which is a stress for me and I find it ridiculous to be threaten with termination when nothing was ever discussed with me about my "misconduct"