"The baby is already alive. The girl shouldn't be 'forced to carry a baby'. No one should be forced to carry a baby. However, people who are already carrying a baby should be compassionately supported through the difficult and tragic circumstance of carrying a child that they do not themselves want.
Wantedness does not define a person's value. There are born children whose parents don't particularly want them, but it is not legal for them to kill those children. The only option in such a case is to relinquish the child to someone who does recognize and validate the value of the child, i.e., someone who does want the child.
There are a lot of people who would love to adopt this 13-year-old's child, therefore the baby is actually not an "unwanted" child, and I dare say that although the term is convenient, I'm not entirely sure that it actually exists in reality. There are waiting lists of parents who will ONLY adopt children with Down Syndrome or other disabilities; there is someone for everyone.
Although what is happening to the young girl is tragic, we should not compound the tragedy with another tragedy on top of it. She should receive intensive, crisis-oriented therapy for the duration of the pregnancy. She should be comforted, counseled, and loved, and the child she carries within should be allowed to continue to grow and live until s/he is born and adopted by parents who want him/her very much.