Yes, the birthmother can change her mind within 60 days of relinquishing her rights, as @crisw says. That is one of the reasons we chose to adopt through Lutheran Social Services. Birthmothers get counseling and support from whenever they contact the agency, to be certain that they are making the right choice for them. If they decide against adoption, LSS teaches the mothers life skills so they can cope with motherhood. Also, birth parents and adoptive parents are encouraged to get to know each other and to interact.
Even after placement with the adoptive parents, the adoption is not final for 6 months so the baby could still be removed from the adoptive home, but not necessarily returned to the birthparents.