First Congrats!! What an exciting time for you! I would definitely ask your social worker for more information (that's what they're paid for), and ask her to send you a copy of the specific laws.
Here is a link to a page with some info about Michigan adoption laws. If your SW is of no help, it has some 800#'s that you could call and find out when parental rights are terminated, and information on when your "guardianship" starts after birth. It appears that the interstate compact in Michigan must be approved by the state before you take custody/guardianship. I may be mistaken though.