This is one of those things that's changed since I was a kid (30+ years ago) that I find quite interesting. I have kids myself, but I've realised that my default position when I meet a woman of 30-40something out of context involving families(eg work) is to assume she doesn't have kids, and I seldom ask unless something they say or do suggests that may be the case
I imagine when I was a kid, most women 25+ would be assumed to be married and have kids as default. Of my long-term mates, I'd say only about 50% have children - we're mostly middle class, educated to uni level, if that might have any effect on how I see things, but I'd be interested to know what other people tend to assume about their female peers in terms of them being parents or not. I suspect it's not something we think about consciously most of the time, but different backgrounds may affect what we presume about other women's family situation.