What you have provided is a bit vague, but here are some questions that you may want to answer either to us or at least to yourself.
1. Is it yours?
2. Are you sure?
3. If it is, what are your legal responsibilities? (seek advice from your chain and JAG)
4. Whether it is or not, what are your personal responsibilities to both the girl and the child?
5. What is the condition of your relationship with this girl? Think long and hard about this. Pregnancy is not a good reason to get married, but it is a good reason to reschedule a marriage. Also consider what it would mean if you do marry, and the child is not yours biologically.
6. What are your current financial obligations, and can they be scaled back? Last time I was assigned to a major installation, there were budgeting classes available regularly; I assume that is pretty typical.
7. Are their any genetic conditions in your family (downs, cystic fibrosis, sickle cell, etc.)?
Army leaders have seen plenty of troops get into bad situations with conditions similar to yours. While what they say can be offensive to both you and your girlfriend, keep in mind that they are looking out for your welfare. Seriously consider advice before rejecting it. This goes for those in your unit, and random folks online.
Your girlfriend will have a difficult time with this, no matter what decisions are made. Research the subject so you will at least have some expectation of what is coming. If you can get some relationship counseling, do so.
Things that are expensive:
1. Medical care
3. Marriage (not to be confused with weddings, which can be rather reasonable. I've been wed twice each costing less than $50)
5. Child support
If she is offended by the suggestion of a paternity test, tell her it's an Army requirement. The Army is a great scape goat (e.g. can't visit the in-laws this weekend, my pass was revoked).