Ahhhh, yes, congratulate her - make her feel like the mommy she is to become. I can't imagine how I would have felt had people told me all the bad and not the good of having a baby. Nowadays, there are a lot of supportive groups for teen moms and options (parenting classes, child care so she can finish school, college, hot lines where she can talk to professionals who will help her cope with being young and pregnant, etc.). My husband and I were very fortunate. I became pregnant at 19 and was already living with his family (due to parental neglect from my parents). His parents, although were kind of upset at first I think, did a lot for us. Thanks to mom-in-law, I got EVERYTHING I could possibly need for my baby (stroller, crib, swing, clothes - just everything). They also encouraged hubby to treat me like a princess (haha - he has done that with every pregnancy I've had)
Obviously, teen pregnancy should be avoided, but if she's already pregnant, no sense in bringing her down about it. Pregnancy should be a happy time no matter how old you are. I think the more support you have during pregnancy, the more likely you will be a great mom!