Bf won't let me confront his mom

I told my bf that if his mom says or does something else that offends me I'm going to say something to her and tell her how I feel. He said "I swear if you say something to my mom we'll have the biggest fight and I'm going to hurt your feelings" 
His mom tries to tell me what I can and can't do with my baby once she gets here,what she has to wear,heck she won't even let me pick up the slightest thing (a few grocery bags),she's even gone as far as making comments about how I'm too tiny and comparing me to other pregnant women and showing me pics of them and saying "you should be this big". There's many more things too. Maybe I'm just hormonal but man she upsets me at times.