There is no simple answer to this, since each situation and relationship is different. What I would recommend in most cases is that the couple wait at least until the child is born to make a decision. There is so much to consider in those nine months that the couple will not be in the best frame of mind to discern marriage.
There are at least four options open to the couple: to place the child for adoption and have the couple go their separate ways; to place the baby for adoption but stay open to marriage; to get married and raise the child together; or to keep the baby and not get married. All four options will be difficult, so it is important that the couple seek the best advice they can find. By talking to parents, priests, and counselors at crisis pregnancy centers, the couple will be able to make the best decision for the child and for themselves. In most cases of unwed teen pregnancies, I would argue that the most loving option for the child is adoption. That way, the child is raised within the context of a stable marriage. Being an adopted child may have it’s challenges, but they’re not nearly as great as being raised by a single teenage mom.