Someone not giving up their for a pregnant lady probably means they haven't noticed the lady is pregnant, not necessarily that they are a rude a-hole.
I think sometimes it's really hard for others to know we are pregnant. Especially when they are busy with their own life and not looking (I don't eye up others when I am on public transport), we aren't *that* far along, we may naturally be on the larger side and/or it's cold and we have layers of clothing on. For every story of a pregnant lady not being offered a seat there's probably one about overweight lady/new mum
Being offended because someone thought they were pregnant.
If someone is pregnant and doesn't have a seat then they need to say something. "Excuse me, would you mind if I have that seat? I'm pregnant and not feeling well. Apologies if you need the seat - I can ask someone else." Is it awkward doing this? Yes. Should we have to do it? Ideally not. However we can't not do it and then express concern about how we don't have a seat.
Good luck with your babies ladies!