How to Control (confront?) an Overbearing Mother. Long post but advice most needed please.

I'm expecting and due Oct 3rd. My mother and I have unfortunately a terrible relationship. Sigh. It stems from very strict parenting, lots of restrictions as a child; adult years, mother was (still is) verbally abusive, condescending, and just all around hard to deal with. 
Anyway, she told me that she plans on flying to OR (she lives out of state) and staying for an ENTIRE month when the baby arrives!!!! The idea of her being there that long terrifies me to no end. I mean by the end of a convo with her I feel about 4in tall. So many of our convos are very forced talks about "how's the weather?" Smh. 
Besides the relationship strains between her and I, there's other problems with her staying on that long. She hates my SO (told me so to my face), so clearly he's not happy with this and from a practical standpoint I live in a studio apartment! My mother says she intends to stay with us because she doesn't feel she should have to pay for her own hotel because "she's family." Brother. 
Anyway, there are numerous reasons why I don't want her staying for that long when the baby arrives. I attempted to explain all my reasons as diplomatically possible, but she interrupted me and accused me of being a disrespectful child who "shouldn't talk to her mother that way."  I'm  sad to say I'm 30 years old and I still get talked to this way by mother. Sigh. Deeper issues present, but current issue is her inviting herself to a monthlong stay not solicited by my SO or I. 
Need advice on how to approach her again?? I was thinking of writing her a letter, but would that be insensitive? So conflicted! I don't want to be  this "disrespectful child" she accuses me of, but I just can't deal with the newness of baby plus the anxiety attacks she prompts (full blown panic attacks that get me hyperventilating) -- she just can't stay! SO says to change my cell number and literally remove her from the equation, but as bad as things are with my mom I'm not sure I could go to that extreme. 
Gosh, how do you lay down boundaries as an adult child with your parent without crossing some arbitrary line that makes one a bad child?