First off, I'm terribly sorry that you have to even deal with this situation in the first place. I would say to stop going out of your way to appease her. She is a grown , intelligent woman if she can't be happy for the two of you then it is what it is. You will never "win" and she is not going to change her ways. I would see how she handles the next steps.
She sounds awfully like my more there. Ill be honest, even tho people will say to try and patch up the relationship as much as you can, I say cut your loses. Inform her now and then of things happening but other then that keep your distance. Its your baby, your body, and she has no say in anything as much as she says otherwise. I'm pregnant with my second as well and this is what I had to do with my mother. She absolutely worked her hardest to b tear my husband and I apart, still is trying to take my son, and just overall makes life all about her. All it was was stress for me all the time. So much stress that i miscarried our first time trying for a second. You don't need that stress. Throw the details out there so she knows but don't go out of your way to keep her involved. She's a very toxic person from the sound of it.
Gosh, I'm a big fan of people getting along with the in laws. I think having as peaceful a family as possible is important even if you have to eat crowe from time to time or shove down feelings of irritation. However, this is different. She threatened to take custody of your child. If you aren't a bad mom or doing something harmful, that makes her a bit dangerous. After that, I'd keep her at arms length. I hate to say that----- but in most legal situations, when you are threatened by another party, they say to have no further contact. She threatened you and your family. Now, she's your man's mom so you can't totally cut her out--- but I would limit involvement or emotion. I would not jump through hoops to please her. And I'd document everything on your own so that should she act on any threats, you have your own documentation to back up your side. Threats made in front of others, anything written, etc. Save it all.
She is being passive aggressive by not saying much. Go on with your life and enjoy your pregnancy. It's sad for your partner that he has to probably see his mom behaving this way. I'm sure he has many wounds from this--- you just be supportive to HIM and not worry about pleasing her. Tell him how much you love him, make a happy home, share peace and joy together--- and his mom can sit in her house unhappy trying purposely to rain on everyone's parade.
Hugs, let us know how this progresses!
When you go into labor or have the baby inform her and let her decide. My mil drives me nuts. She is upset about the fact I'm pregnant again and her daughter is having issues getting pregnant. So they just don't talk about my pregnancy when the daughter is around. It's werid.