S.B.
Dear K.,
Wow. You are in a tough position. I really feel for you especially since you are expecting another new baby any day. So.....I'm afraid you are going to have to have a talk with your mother before the new baby comes or there will be even more conflict which is the last thing you need with a new baby. And the new baby doesn't need it either!
I would just come out and tell her, "Mom, your words really hurt me. I need you to know that. I'm your daughter, and I love you, but the way you criticize the way I discipline, when you criticize my husband, when you make me feel bad, IT HURTS ME. My husband, who you can't stand, added another story to our house so you could come to be with us, and you don't appreciate any of it. I've heard you say a million negative things about him but not once have I ever heard you say THANK YOU. I've got another baby coming and I don't think I can take this anymore. You are wearing me down and I need all the strength I can muster to adjust to a new baby with a 2 year old. We brought you here so you could be a happy part of our lives but instead, you hurt me with your words. Something has to change. You don't have to agree with all my decisions, but you have to let me make my own way. I'm a woman now, Mom. I am a woman, a mother and a wife. I will always be your daughter, but I'm not 12 years old anymore. Please don't treat me like a child. It hurts me. I would like to be able to ask your opinions about things, but I know you're just going to say something hurtful and negative. I don't need that. What I need is some support from you. I don't need a daily list of all the mistakes you think I make, I need some encouragement. Please."
If your mom cries and tries to make you feel guilty for spilling your feelings in a concise and calm way, or tries to blame your husband for telling her how she is hurting YOU, then the next step would be to find somewhere else for her and your dad to live. Someplace close by so they can visit regularly but just not have that turmoil in your own house 24 hours a day. It's not healthy. You might also ask her if she is unhappy with the decision to move here or with the decision to live in your house. Maybe she's not happy and being a complete grouch to you about it. In which case, again, find your parents somewhere else close by to live.
Your husband will get fed up, you will blow like Mt. Vesuvius if you don't get this off your chest, and it's just not a good situation. You are obviously a kind, respectful and loving daughter, but I wouldn't worry about hurting your mom's feelings at this point. She's been hurting your feelings and she needs to know that. Before the new baby arrives.
I wish you the best and let us know when you have your baby.