You’ve made a good start. You drew a boundary and stuck to it. It sounds to me like your mother may be competing with your husband for your time, attention, and love. Is it possible that she feels there isn’t room in your heart for both of them? If so, the problem to address is her insecurity. I wonder if the situaiton will get better or worse once your child is born. Sometimes the desire to be part of a grandchild’s life makes someone like your mom start to behave. But if she is worried there isn’t enough love to go around, she may compete with the child too.
If you think she’s insecure, try to work on that problem directly. Make sure to give her attention when she is doing things right. Give her random “I love yous.” Catch her and praise her whenever she is doing something right. Reassure her that you can love both her and your growing family.
If, on the other hand, she’s just one of those mean-spirited people who targets others, keep doing what you started. Make it clear what you will and won’t accept and mean it. Don’t argue. Arguing implies you can be talked out of something. Simply state clearly what is and isn’t okay. When she acts up, leave without further comment. You’ve already told her what’s unacceptable so engaging in yet another conversation about it only gives her attention for the wrong thing.
I hope your mother responds. Life is too short. She has the opportunity to have a loving relationship with a daughter, her son-in-law and their children. What a shame if she can’t appreciate that.
I wish you well.