One of my students once told me about an experience she had while riding the train. She was sitting with several other people when a family of three, mother, father and child, boarded the train. The woman from this trio started loudly sniping about how no one was forfeiting their seat for her despite the fact that she was displaying a badge that indicated that she was pregnant. Like many people on the train, my student was in her own world and didn't notice the badge. Indeed, it was hardly on prominent display, and the woman was not visibly pregnant. While I realize that some pregnant women may experience difficulties when their child is still embryonic (or indeed a few cells), I think that a lot of women use these badges to bully people on crowded trains into giving up a seat. In fact, I'm not entirely convinced that all of the women showing them are actually pregnant. To be honest, if you aren't visibly pregnant (i.e., not carrying much in the way of extra weight) and too fragile to stand on a train, perhaps you're too delicate to be on public transportation at all.
I won't miss the women who act aggressively entitled to a seat because they are displaying these badges when they are not at a stage of their pregnancy which requires special accommodation.