Juliet is Claudio's baby mama. In the play, her pregnancy is what tips off Angelo and the authorities that she and Claudio have broken the law by having sex outside of marriage.
If we're expecting Juliet to be branded with a scarlet letter (like, say, "F" for fornicatress), we're definitely reading the wrong piece of literature. Instead of being punished like Claudio (who's sentenced to death), Juliet is sent away by Angelo to some place where she'll be safe and healthy, but not too comfortable. (Read: prison.) In fact, we don't see much of her until she's trotted out in the final scene where the Duke announces that she and Claudio can be together.
That said, Juliet's minor role in the play makes a big impact. The fact that she gives birth in prison is a glaring reminder of the government's attempt to regulate sexuality and biological reproduction. You can read more about this in "Themes: Justice and Judgment."