Some of those who were good friends will vanish into the ether, people who should know better will cut you off, meanwhile folks who were acquaintances will become Godsends, and people who have been less than social will suddenly be offering you company and help - and not because they want to see the baby.
I think it's horribly dismissive to suggest that those that don't have kids just don't understand, especially when I've seen people who have children show way less understanding than some that don't. Yes, if you don't have kids you don't know what it's like, but that doesn't mean you're stupid, or lack empathy. People are people, some get it, or at least try to, and some don't, and don't care to try, and it's got nothing to do with whether they've pumped out a sprog.
I recently tried to visit a friend of mine, a single mum living with her parents. As much as she wanted me to visit, her parents wouldn't have been receptive to visitors (anyone, not just me), and so instead she ended up collecting up bub, hopping on the train and meeting with me that way. Life would have been much simpler for her if I'd been able to visit, and one would hope that her mother, a woman who has had a few kids, would understand how nice it would be to have someone happy to come and visit. But no.
Some parents are preternaturally thick when it comes to people not themselves. Like the woman who, when ever someone would mention their premature baby, would immediately talk about how hard it was for her. Her baby was a couple of weeks early (which is medically considered full-term) and she acted like it was a terrible strain to have to deal with one so young. And she'd be saying this to people whose children were born so early they were in humidicribs for months and unable to breastfeed because they were too small and didn't have the strength!
Sharon and I have been lucky, we've only had one person seriously distance themselves from us, and we've had other friendships grow and blossom. With those we've been pleasantly surprised, and are happy to see them grow.
So be prepared for it. Know that relationships are going to change with people, and don't begrudge those who distance themselves. Just try to offer them some understanding, and in the really extreme cases, forgiveness.