I'm going to chime in - I live in an urban area with lots of public transit which I utilize. When I was pregnant I was given a seat exactly once in my well over a hundred times I traveled it. I could have really used a seat in my first trimester but I looked young and 110lbs soaking wet. With no 'baby bump' who was to know that I was sick and exhausted. Even when I was huge people looked the other way except for one teenage boy, once. He gave up his seat for me on the back of the streetcar. He was shocked by the 50 other people I had walked by that didn't offer. I wasn't, it's the way our society seems to work here in North America... when I was on a tram in Switzerland I once saw a young man with a 1.5' mohawk jump up, and help an able bodied elderly woman up the one stair of the tram, walked to her a seat and held her groceries until she go off. Scenes like this were echoed multiple times daily while I was there.
Not saying one is better than another but one is definitely more thoughtful and selfless.
Looking at your location, we are using the same streetcars. I'm horrified that you never got a seat - rest assured that you would have had mine. Okay, probably not in the first trimester, but otherwise.
No, I don't that anyone should expect people to give them a seat or feel entitled to one, but I offer my seat to anyone who looks like they need it more than I do at the time (I have Lupus and other connective tissue issues and, occasionally, need the seat more than it may look like I do). This is especially true at home (i.e. city transit) where options like leaving the park before your child is too tired to stand, do not exist.