A case study is when you explore the fact, the reasons and the results of it. You have teenage sex and pregnancy, so first you should give a background. You include the teenager’s age, the situation in family, whether the teenager gets education, etc.
Then you state the problem and the effects of it. And you also give some possible solutions to it.

To do case study you need a story, that is the case, you read a real-life example of, say, a pregnant underage schoolgirl. You have to know her situation to the letter, the reasons and the problems she had when pregnant, and if anyone helped her to cope with it then who did so, was it her family or friends, and so on.
At any rate you’ll have to adjust the study, I just give you structure.

Mind that a case study should not overview a stationary situation, but the situation of a person or group of persons over a period of time. You make a thesis and rely on an example of this very situation to demonstrate how this thesis is correct.
You do teenage sex and pregnancy, then first you should formulate the thesis, it can be reated to, i.e., short-term and long-term health issues in teenagers who had sex and got pregnant too early, etc.
Then you choose a real situation and make sure you know all about it, especially in relation to your initial thesis.