We told my mom, as she's coming up for the birth.
I think we probably eased into it a bit (though not purposefully) just b/c of how OUR decision came about. I was upfront with Mom very early on about my desire for UC, but DH wasn't on-board. So we saw a midwife and DH and I kept talking about it. Meanwhile, I kept talking to Mom about just wanting a HB (and, since there are no HB midwives in our area, that basically meant UC).
At first, Mom was worried and glad that we were going with a midwife. But as time went on and I spoke with her more, she started to be less "pro-professional", so to speak. At the same time, DH was becoming disillusioned with the midwife.
So eventually, DH came around and then we told my mom. She had seen the whole progression through our conversations, and she also saw that DH (who had been against UC) was now very much on-board. So she was able to put her personal beliefs/preconceptions behind her and listen to what *I* needed her to hear.
I think she'd still prefer we birth at a hospital or bc, but she knows that those are not options for me. She won't say anything about it, especially since I've told her that I do NOT want to hear her fears (well, actually I told her that after a certain point in my pregnancy, I did not want to hear about anyone's fears, and she never brought any up).
I also addressed my concern about her "vibe" messing with me during labor. I was very clear and told her that while *she* may not necessarily believe it, *I* do. I told her that if she was going to be here for the birth, I needed her to leave her worries/fears/concerns/"helpful suggestions"/etc in Kansas and NOT bring them into my house. I also told her that if at any point, she felt she had a legitimate concern about how the labor was progressing, she was to take it outside my house and really examine her concern to see if it was a TRUE concern or a false concern (ie, "taking too long", "too much pain", whatever).
I think my mom sort of thinks I'm goofy about this, but I'm taking it seriously. I gave away my power during DS' birth, and I will NOT do that again.
But I think my mom was "convinced" in part by seeing how DH and I made our decision. She knows that we have considered the "what ifs", so she doesn't feel that she has to bring them up. I've also gone over and over DS' birth with her, so that she realizes how traumatic it was for me, and how a hospital birth (barring emergency) is NOT an option. Basically, my mom's got my back - even if *she* wouldn't make this particular choice for herself, she respects and supports my right to make it for myself.