singaporeans can be very unreasonable, especially when they are in certain conditions (pregnant/ elderly/ etc); they think they have certain rights that others must adhere to- rubbish...
with regards to pregnant ladies, there was this once when a pregnant lady kept pointing to this corner seat where there's a 'give up your seat to those who need them more' sticker. the person in that seat was genuinely sound asleep, with his mouth agaped but the lady kept grumbling that the seat was 'reserved' for pregnant people so it goes to show the individual's understanding of her rights.
pertaining to the original post, the lesson learned here would be, if you see ladies with protruding tummies, ask if they are pregnant: Excuse me madam, are you pregnant? Would like to have my seat? if she gets offended because she's fat & that's her belly jiggling while the train's moving, then it's not your fault. feel good because you are being civic minded.