I think calling a pregnancy 'unplanned' is stupid. Are you telling me that kids don't know you can get pregnant by having unprotected sex? If you have sex, then you should be either using two types of birth control at the same time, or planning on getting pregnant.
Yes, the baby will need things when it is born - so if a friend wants to give something to help out, sure, go ahead, but don't play the game that the young, incapable mother is a victim. She made a choice, and I wouldn't be celebrating that poor choice with cake. (I would go and tell the stories about 48 hour labor, back labor, etc.)
I have worked for over 10 years with special needs students, and a majority of them come from families where the mother was undereducated, incapable of even supporting herself, and made the choice to have a family at a very young age. There are no fathers in the picture, they rely completely on government support for all their needs, and they can't get out of the trap they got themselves in. I don't see that as a reason for a celebration.