How to tell my boyfriend I'm leaving?

Dont tell him you are going, just get your parents to come over when he is out for a while and pack quickly and go. Telling him you are leaving could put you at risk if he is abusive or has the potential to harm you which sounds like he could from what you wrote.

my mom said I have to tell him before I leave.

I would leave while he’s at work or out somewhere for a longer period of time. Don’t tell him, that might set him off. When you’re somewhere safe, call or send a text letting him know you’re done. Me personally, I’d send a text.

You should tell her it might escalate to a fight and the safest thing is to not tell him and just leave.

No, tell him after. Especially if you feel unsafe to do it at all.

Just tell her you did but don’t until you’re out and safe

He'll be at the house when she comes up so it would be better if he knows before that. She would know if I didn't tell him. I tried telling her that it would escalate to a big fight if I told him and that I've literally tried to leave before but had to back down because of how verbally aggressive he got. She said "this isn't going to be a secret."

He's never been physically abusive, but he says awful things and yells/swears at me a lot.

Why is she insisting on you telling him before leaving?
The priority is you and your unborn child being safely out first.
No one would advise you to tell him before leaving, as it could - and from what you’re saying probably would, put you at significant risk. Any increased risk is definitely not worth it.
I don’t understand why she would want you to tell him first, especially at 29 weeks pregnant.

I honestly don't know why she is insisting I tell him before

Do you think she is concerned you will change your mind after speaking to him??? The optics of that aren’t great and I’m sorry that you’re going through this.

I think he’ll get the hint when he comes home and finds out that all your stuff has gone. Take the light bulbs and ice cube trays with you as an extra ‘fuck you’.

we live at his dad's house so I'm not taking anything that isn't mine. He'll also be gone when my parents come to move my stuff.

I don’t get why your mum is so adamant that he needs to know. For a start, it sounds like he’s had plenty of warnings. And she should be aware that the most dangerous time for a woman in an abusive relationship is when she tries to leave. I would wait till you’re fully outta there and drop him a text.

May I suggest the following wording: “Bye, bitch.”

my dad will be here before my mom by a couple of hours. I think I'll call my dad tomorrow and see if he is okay having me wait until he's there for me to tell my boyfriend I'm leaving. My mom also keeps asking if I'm sure I'm done, if I think I'll change my mind, and keeps telling me to let her know if I change my mind before they come. I don't think she's ever been in an abusive relationship, but she didn't like my boyfriend anyway so I'm not sure why she keeps saying stuff like that. I've told her a bit about what's been going on and that I've been thinking about leaving for a couple months.

I hope you know that I meant the optics of your mom thinking you will just stay after you speak to him are bad (and not that anything you’ve said looks bad). I would not tell him until you are out OR if other people are around to keep you safe. Sending you love. You’ve got this.

I figured that's what you meant. I would be telling him with his sister and brother in law where they can hear in case something happens.

I would feel better if you had someone entirely in your corner there.

I just know he would get more upset if he knows I'd been talking to his sister and brother in law about it and that they already know I'm leaving. Their room is right next to ours so they'd be able to hear the whole thing.

Honestly unless it’s something you ABSOLUTELY must have I’d say pack what you need and Important documents and dip that’s what I did a few years back I left so much stuff behind and I honestly don’t think about any of those things till this day