What she said. But will add more.
Birthmothers (and fathers) choose the baby's parents (especially in infant adoption) and they work out a plan with the adoptive parents about how much they will be involved in their child's life. For example: a birthmother may choose not to have any more contact with the parents after she finds the right family for her child and another birthmother may choose to receive letters and pictures every so often and perhaps a visit everyonce in awhile and yet another birthmother may be very involved and considered part of the family.
Open adoption and closed adoption also works with family members, a open adoption helps children remain in contact with grandparents, siblings, etc.