When not pregnant would always offer but if they say no I wouldn't feel guilty - some people have been sat all day and prefer to stand. Now am pregnant I always take a seat if offered, and if I saw an old person who thought needed a seat would be more likely to ask someone else to move than move myself (have never been in situation where everyone is more in need than me), is not that I can't stand but is more that trains and trams in particular often jolt and judder and don't break smoothly - other people fall over into you etc. so it is a stability thing and risk of a fall why I sit rather than an issue standing. I always ask if people don't offer and I think most of time people just haven't realized rather than are being rude - and have had opposite experience to PZ, in that often have many people trying to give me their seat, or old ladies instructing others to get up!