In my opinion, a lot of teen pregnancy is caused by carelessness on the part of the teens in question. Another large chunk of teen pregnancy is purposeful, either "planned" or "accidentally on purpose".
The reason I believe this is that any teen who truly did not want to become pregnant and have a child, didn't have to do either. Choices were made along the way that led to teen parenthood. Most teens with children have them because they CHOSE to have them. No one if forced into childbearing anymore.
Only a tiny percentage of births to teens is not on purpose. An example would be the teen who truly got pregnant accidentally, and did not realize they were pregnant for many, many months, until it was too late for any other course of action.