Need advice! Term pregnant bridesmaid?

So just found out I am pregnant with our first😍😍...estimated due date 9/27...but I was asked to be a bridesmaid a couple weeks ago and her wedding is 9/15. I feel like I should let her know sooner rather than later bc those short tight fitting and unforgiving bridesmaids dresses will NOT be an option at that point. And there is always the possibility we could be a little busy in labor! Any thoughts?

**we aren’t planning on telling our parents til 12 weeks bc my MIL can’t keep a secret to save her life...