A perfect storm of government-mandated family size and a secretive government which withholds sex information from the young.
Apparently, contraception is difficult to come by as well. So those lucky few who know to take steps to avoid pregnancy can't take those steps.
This jumped out at me as well: Nearly half of Chinese women seeking abortions used no contraception. Which means that 6.5 million women in China used contraception but still got pregnant, which sounds like a real educational problem.