If someone is clearly elderly / pregnant / on crutches or something then of course I will offer them my seat.
This morning there was a woman who I am not sure if she was pregnant or just a little chubby round the middle. Hard to tell as she was wearing lots of layers with it being winter. She wasn't wearing a 'baby onboard' badge or anything which a lot of people do in London.
In this situation AIBU to just not offer seat and assume that if they need a seat they will ask? Just don't want to risk offending anyone by suggesting they are pregnant when actually just a bit chubby.