http://www.cnn.com/2008/US/05/04/teen.birth.ap/index.html

What I don't understand is how does a parent not know a teenager is pregnant until she delivers the baby?

Some women don't show hardly at all. On top of that, this girl appears to be a larger girl so her mother may have simply assumed she had put on some weight!! If the girl was careful enough, and the mother paid little attention, it's not impossible!