There is a thing called "Belly Etiquette". Seriously. Google it and you will find 51,100 websites dedicated to the subject matter.
I remember with each of my friends pregnancies, they all had the random stranger approach them and rub their belly (without permission). Some would even say they are rubbing it for good luck, which, by the way, is ridiculous. How is rubbing someone's belly going to give you good luck?
According to nestbaby.com the #1 Belly Etiquette Fauz Pas is: “LET ME TOUCH YOUR BELLY!”
"We’re all guilty of the belly rub… it’s hard to resist. But hold back unless you ask the mom first. Not only is it scary and weird when someone, whether it’s a stranger or even a relative, touches her, but a woman’s belly is private, and she may not want you reaching for it."
I had my first experience today with someone approaching me and making a very poor attempt to rub my belly. Unfortunately, this person didn't realize that patting it as though she was patting her dog after doing a trick was not considered appropriate, let alone gentle! Baby says, "Ouch!"
Thankfully, most of the people in my life already know some form of belly etiquette. They know what to say (even if they haven't been pregnant before), they throw out the appropriate compliments, and they'll even ask before rubbing (not hitting) my belly.
Just remember - pregnant belly's draw attention, but with that comes etiquette. Or, at least it should...