PIP is not about performance. It's HR formality and prelude for firing. It's the way HR uses to protect the company from law suits. Your managers and HR are not your friends, they want you out. Most people who are put on PIPs ended up being fired, or they quit, very few survived a PIP. Even if you do survive it, your manage can put you on PIP again the next review cycle. I know it's hard but try not to take it personally. They probably have to lay people off. So they put people on PIP and force them out. This way, the company doesn't need to give you a severance package and they avoid law suits. plus they put employees to shame and people tend not to talk about the experience.
In general, your best bet is look for another job and quit the current one before your PIP ends. But since you are pregnant, I am not sure how that works (I don't live in the US, not sure about the US system). Is it possible to get a doctor's note and get on short term disablity till you deliever. Then take your matt leave, then look for a new job and quit this one during matt leave. You probably want to talk to a lawyer. I am not sure, but I think you should have some extra legal protection agaist firing because of your pregnancy.
Best and good luck!
