Becoming a mother has profound implications, not least of which is the re-introduction to social pecking orders. Not since i was in highschool have i been thrust together with groups of women that i would normally not come into contact with... ever.
In highschool (and i'm basing my comments here on a public school experience) you have no choice about the people with which you are confined to a building day in and day out . Students do what they can to mark themselves as part of a group, but it's impossible to completely avoid the jokers plaguing your yearbook class.
College provides a much larger pool from which to find kindred spirits. Move along the non-profit route and generally you find yourself surrounded by people that share many of your social, intellectual and cultural markers.
One day you're blithely mocking urban hipsters (welcome to the world Henry!); the next you're sitting on the floor of someone's living room exchanging parenting tips with a woman in oversized sunglasses and manolo blahnik boots.
True story, a group of us with babies 6 months or less are gathered in a neighbor's living room exchanging stories about meltdowns our babies experienced while we were out. The woman across from me began by recounting the time her newborn son had a fit while she was shoping in Sacks 5th Avenue. It didn't even occur to me what a contrast my story of Henry's freak-out in Walgreens was until well after the fact.
All of this to to say that motherhood is a much stronger forced commonality than high school district ever was. The fact that you can make absolutely opposite choices about 99% of your life and still find a fierce 1% of parenting topics to hash over with strangers is a really new experience for me.