I guess it’s a matter of culture. In my country (Switzerland), people are used to ask for a seat if they need one. Giving up your seat if someone is not asking for it… we just don’t do it. Some elderly even get angry if you offer them a seat they didn’t ask for.
So… why not just ask for a seat if you need it? Not everyone wants to sit, regardless of how pregnant or elderly or disabled they are. I’m sure nobody will deny you a seat if you ask for it.