Why do teenage girls get pregnant?
Well, most of the time, they don't do it on purpose. Either they don't know the facts about sex and birth control, and they mistakenly believe they can't get pregnant when they can (like, they think you can't get pregnant the first 10 days after your period, for example, or they think you can't get pregnant if the woman is on top during sex), or they just don't believe it could ever really happen to them.
Or, they use condoms, but the condoms break. Or, they forget to take their birth control pills. Or, they pass out drunk at a party and get date-raped. Or they get raped some other way.
And some teenage girls do get pregnant on purpose. Usually they have emotional problems, and they think that having a baby will solve these problems. Maybe they never got enough love and support from their parents, and they think that if they start a family of their own, they can replace what they never had. Or they are in love with a boy, and they think that getting pregnant will make him stay with them and get married to them. Or they simply have absolutely no idea how much work and responsibility having children is.
It has certainly increased. From what I have learned in health class, around 1 million teenage girls become pregnant in the U.S. every year or 1/3 of young women become pregnant during their teen years in the U.S.. During the late 1980's-1990's it certainly was not 1/3 of teenage girls becoming pregnant.