The main reasons why young women get pregnant, if it is deliberate is because they want to show that they are grown up and are independent, and because they are lonely.
I tend to think that many do not intentionally become pregnant they do not get a prescription for some birth control device because many see this as slutty behavior, and they do not want to feel slutty.
My students were high school graduates, and generally between the age of 18 and 24 when they had their first child. Every semester I had at least one that would show up in the first week or two of classes and vanish until final exam week, when they would appear in my office and ask what they needed to do to pass or get an incomplete grade. Unfortunately, I could neither pass them or give them an I. All of which I explained in the course syllabus.