I am really sorry you are dealing with this.
You are an adult, and can tell your mother whatever you want. You know her best and can probably best predict what her reaction is going to be. Perhaps if she hears from you that you are not comfortable being a part of things, she may reconsider her decision and have her wedding at a more convenient time. If it is important enough for her to have you be a part of it, she should.
Also, just my two cents, I would not be driving 2 and 1/2 hours each way being a designated driver while I was 38 weeks pregnant. I wouldn't even do it now at 12 weeks. Plus, being 2 and 1/2 hours away at 38 weeks is risky. What if you go into labor then? Many women choose to stay pretty close to home when they are that far along.