My pregnant girlfriend is trying to leave Nevada and move to Boston. Can I stop this?

Short version: no.  Until (and unless) a child is born, the custodial sections of the UCCJEA do not not apply to "custody" decisions.  For details, click here.  You COULD initiate a paternity action which also will get stayed until the child is born (details here), but Nevada would retain jurisdiction over that determination, if not the subsequent custody action post birth.  Nevada could also adjudicate support issues.  For explanations of all of this, see the article "The Basics of Family Law Jurisdiction," posted here.  Really, you should consult with a family law specialist about the full range of your options, risks, benefits, and obligations.