So, I can speak to this from a couple perspectives.
My MOH was pregnant and told me maybe a week before the wedding (at 3 months) and another bridesmaid was waiting on an adoption in which the birth mom hadn't chosen and was due 3 days before my wedding, so there was a chance she couldn't make it at all. My bachelorette party and bridal shower were both planned for the week of my wedding. My MOH did everything she could that didn't involve drinking and still came out with us to the bachelorette party. She ate food and had fun as far as I could tell. The other bridesmaid's baby came 9 days early and she managed to come (and the birth mom chose her, yay!)! It was the happiest time in these women's lives. Things changed for them and yes, I was a little shocked at both of these situations and cried a little. It sounds like your bridesmaid might not be certain of her role at this point. While the two ladies in my wedding both came in with a plan, yours just might not be that organized. Engage her. Show her maternity dresses (she might want one if she's post-partum) or how to size up for one, ask her thoughts, go shopping with just her if you have to. I think you might be leaving her in the dust on all these things when instead you could be proactive and say, "Well we can look at maternity dresses for you or ask the consultants what works best for women a month after birth. I don't know, but let's figure that out together," or, "Is there something we can do at the bachelorette party that would be fun for you too? I want you there, and at the same time understand if you want to call it a night a bit early." Communicate and compromise. She's your friend.
I'm in an upcoming wedding. I'm TTC. The bride knows. I've ducked out of the bachelorette party (destination) because it's an expense I don't want to make if I also end up having a baby, needing fertility treatments (a very real possibility given family history), and because we're trying to buy a house. The biggest weight off my shoulders was telling her this and going, "I'm bummed you won't be there, and I GET IT. You do you and just be at the wedding." What a relief. There are other things I can do for this woman besides attending this party. Crafts, chatting weekly, and whatever. And I can do them pregnant or not, rich or poor.
Don't lose your friend over her life event. Be flexible.