I did get one, and kept it in my bag, but TBH I never felt I needed to wear it. Couldn't work out what the point of it was.
I had a good pregnancy and always felt able to stand, and sometimes needed to after a day at work. Had a small bump so never realy got offered seats as you couldn't really tell I was pregnant till quite late on, and never could tell from behind.
I don't really think they achieve anything that just asking for a seat couldn't. I think that people get unnecessarily huffy about people not offering them a seat when they are pregnant, but personally I feel the onus to ensure you get a seat if you need one is you, not someone who is sitting down reading a book and not paying attention to whether every woman is wearing a baby on board badge when they get on the train or not.
But saying that, if you feel you need to wear the badge to justify asking for a seat, if that gives you confidence to do so, then wear it from whatever point you want to.