How would you react? Read description

So a while ago I was with my partners mother. Anyway we told her that we are trying to conceive but we are not catching on. Anyway I said to her I have been tested and all appears fine but bf needs to go get checked. Anyway she turned to me and said "well my son is not infertile" I seriously had to put my hands in my pockets so she couldn't see my clenched fist. I bloody hate her now. How would you react if your partners family member said something like that to you?