One of the things that is hard to adjust to when you're expecting a baby is that your body enters the public domain. People feel free to say things to you that they would never, ever say to someone who wasn't obviously pregnant: "wow, you're huge!"; "you look absolutely miserable"'; "you're even more swollen than last week!" People also take a kindly interest in your story: I often joked that I needed a t-shirt that explained my stats. 36 weeks. April 7. Girl. Vicki Jo. Yes I'm fat and miserable. That way, people wouldn't need to stop me and ask about all these details.
My theory is that this interest we (as a society at large) take in pregnant women is a sort of indication of the shared responsibility that we feel for raising children in our culture. It is for this reason that I didn't get too irritated with people who wanted to know more about the child I was having. Our culture seems to thrive on radical autonomy rather than community, so I'm all for encouraging the impulse to care for another as much as yourself. We tend to see ourselves as separated (as in, "your health care problems shouldn't be mine to pay for," or something like that) and the things that we do as having no effect on others. But we still care about our babies.
We see babies as our next generation - as the ones who may save us from some of the messes we got ourselves into. So we have an interest in making sure that they are produced in the best possible conditions. Although not many people randomly touched me on my belly when I was pregnant, it probably would have made me mildly annoyed if more did. But I would also have been a bit glad to see that there are still some things that we can all agree upon.