I offer a seat, but it's fairly neutral--I don't feel more protective or tender than I do towards someone who's carrying a cane or visibly older than I, and I offer them seats, too (bus or at an event).

Hmm, fairly neutral in general? None of the bullet points really pings. Some of them might if I knew the person and/or why she's pregnant (by choice or not), but random sighting = I certainly wouldn't ask about that.