You should have a frank discussion with her and ask her what she wants to do.
There’s no reason she needs to resign from your bridal party. They make maternity style bridesmaid dresses and she can sit in the front row instead of stand at the altar if she wants to.
But if she’s due the week of your wedding she may not want to take the chance of spending hundreds on a dress and accessories if there’s a good chance she won’t be able to make it to the wedding.
And just because she decided not to be in your processional doesn’t mean she can’t be involved in the bridal shower and bachelorette party.
When you bring up the topic don’t make it about your wedding, but rather her comfort level. And if she wants to remain as a bridesmaid don’t discourage her. Make a backup plan for who the groomsman she is scheduled to walk with would walk with if she’s not there - remember, he can join another pair of bridesmaid and groomsman