Once an adoption has been finalized according to law, the placement is considered permanent.
In some states, signing voluntary relinquishment papers is followed by a period of time during which the birthmother can change her mind. Once that period has expired, her parental rights are terminated.
Exceptions arise when all legalities have not been observed, or if coercion or fraud can be proved. In these cases, even after finalization, the adoption could be reversed. This is one reason why all parties to the adoption should make it their business to understand the law, know the professionals with whom they are working, and be sure to their satisfaction that the entire process is being done with close attention to legal details.
For more information on the laws related to adoption, please visit Laws.Adoption.com.