Scenario: You are congratulating yourself on having snagged a seat on a crowded, early morning Tokyo train, when out of the corner of your eye you spot a woman standing nearby who appears to be pregnant.
You know you should offer her your seat, but she's a little too far away to call out to her without attracting attention, and it could turn into an embarrassing scene.
Plus, is she really pregnant? How can you know for sure? You can't get a good view of her bag to see if she has a badge that confirms it. And what if you offer her your seat, and it turns out she's not really pregnant at all? What if she thinks you're just suggesting she's overweight?