I've always figured that most of us were unexpected. It's pretty easy to fuck up when you're trying to have safe sex and even if you're perfect nothing short of ripping out your womb is 100%. I know that my parents were pretty young when my sister was born, I can't imagine that they were itching for a kid, much less a second one. Unless your parents were in their thirties, settled down, and had been together for years then I doubt they were the mindset to actually work on conceiving a child.
I'm pretty sure that most of the time people aren't too upset by the idea of a child and that most of us weren't a coin flip away from being soup in the content baggie of an abortionists vacuum and that our parents look at us every day with resentment as they're reminded that they threw away the best years of their lives for a few minutes of awkward condom free sex.
At least mine don't.