Hmmm not sure witj taking The Man - I guess it depends on the conversation and the org but I dont think he would serve the purpose you need ie an impartial person (ideally a professional who deals with this)
Just write it all down, dont go in as though you are ready for a fight... listen and write down what he says and tell him you need to think about it all.
If it is about you being pg that is one thing, if he has issues with your performance that is another. he needs this documented in any case.
Best of luck.