I say this not trying to put you off, but because I've been there.
Don't make any decisions now.
You're having a baby! You get maternity leave! You'll be gone for several weeks, away from her crapola, and you'll be getting to know your beautiful baby. I know you're mad now, and you have every right to be, but you have a wonderful break coming up.
I really think that once you're away from the environment, you'll have some clarity as to what you want to do, and how you want to handle things. You haven't quit yet, and in all likelihood, even if you do quit, it won't be for several weeks or a few months. Don't stress about it now. Have your baby, get to know your baby, and then figure out what you want to do, and if you want to go back.
Although, I do agree with really thinking about not burning those bridges. If you feel like you have to say something, think about what you say, and try to phrase it in a way that says that YOU need something new (implied: that the company can't provide anymore). I can't think of a good example for you right now, but I'll post one if I think of one. Example from me: I worked my butt off for this one company. With the work I'd done (seriously above and beyond my job description or what was expected), I thought I'd be a shoo-in for a promotion. Mind you, it was a promotion in name only....level 1 to level 2. BUT it had a (somewhat) significant pay increase associated with it, automatic b/c of the salary range for the new position. And that was important, b/c this company was not known for paying competitive salaries. No mention was made of any promotion at my review, so I asked about it right afterward. My boss hemmed and hawed and it became very apparent that she FORGOT that I was eligible. Then she blew me off, and told me that she'd reevaluate me in six months. So that was six months that I would NOT be getting the new salary for the promotion that I felt I was entitled to, just because she forgot. At that point, I started looking for another job. I had one within two months. My boss was SHOCKED when I resigned. I SO wanted to tell her it was because she blew off my well-deserved promotion. What I actually said to her was that after two years, I realized that my potential for advancement or new opportunities was pretty limited at that company, and I felt that I needed more so I needed to look elsewhere. It was really the truth, but it was formulated like it was more of a problem with me than with them. However, if she read between the lines, she could see it. And of course, I knew that they just wanted to get the cheapest possible person to do the work. I'd gotten some good experience, but I wasn't content being the cheapest person anymore. So I left.
Sorry for the book, but seriously, really use that gift of your maternity leave. Downtime often equals clarity.