Some prominent sociologists argue that teen pregnancy, when it occurs, is functional.  Teen pregnancy is a foolish life choice for middle-class teens, because they’re sacrificing bright futures.  Lower-class teens, in contrast, don’t have bright futures to sacrifice, so why wait to become a parent?  I’m skeptical of the underlying counter-factuals, but never mind that.  Frank Furstenberg’s “Teenage Childbearing and Cultural Rationality” (Family Relations, 1992) rebuts the functionalists with a thought experiment that is as powerful as it is concise:

[I]f they had to take a pill for a month in order to become pregnant, relatively few teenagers, especially those of school age, would become parents. And, if they had to obtain permission from their parents to take that pregnancy pill, very few parents would give their consent.

In other words, the main source of teen pregnancy is just impulsiveness.  If youths act on their immediate feelings, pregnancy swiftly follows whether they want to get pregnant or not.