Please don't shame individuals into giving up their seat.
I look the picture of health alot of the time but I have a heart condition, an average blood pressure of 84/56 and I suffer from Anaemia most of the time thanks to frequent heamorrages that have never been resolved. If you shamed me, as I have been in the past, I would probably stand out of embarrassment but could easily end up fainting or becoming tachycardic because standing for long periods and being in a warm environment are triggers for me. I feel shit enough about my health without being shamed by members of the public.
I've had the whole "get up, you're a young thing and my legs are older than yours", being tapped on the legs with walking sticks, evil looks and passive aggressive comments from pregnant women. If you want a seat, ask generally, politely and without the intention of being as cruel or entitled as you come across in your OP.
Also, on a good day when I didn't have far to travel, I offered my seat to a woman in her early thirties who looked unmistakably heavily pregnant, was standing with her hand on her belly and was stroking it absentmindedly. She was generally slim but had an enormous "baby bump". She declined the seat offer but we chatted and she was lovely, we discovered we worked in the same field and had a similar cultural background. I very nearly asked when she was due and if it was her first baby etc but didn't want to be intrusive.
As fate would have it, the following night I was walking back to my hotel and I saw the exact same person standing outside having a cigarette and crying on her mobile phone to her boyfriend saying, "I'm just sick of it, everyone thinks I'm pregnant, I can't buy clothes to fit me anymore and I'm in constant pain and worn out with it all." She was in London to see some Gastrointentinal specialists at a teaching hospital and she was having no luck finding out what was causing her symptoms. She'd been investigated for cancer and other serious illnesses. She honestly did look unmistakably pregnant and after that I swore I would never enquire about a pregnancy or pass comment at all unless the other person brought it up first.
Many people have had similar experiences of mistaking a pregnancy so might be unlikely to offer a seat for fear of causing offence. You have to ask. That's just the way it is I'm afraid.