Biological mothers who do not want the baby definitely send signals to their unborn child. I work with children who absolutely do have some prenatal memory which affects their sense of themselves in the world. The mother who sees the fetus she is carrying as a curse or a burden has created a role for that child, the role of the unwanted burden.
Most parents of adopted children who have not gone through appropriate therapy, know how often your child will do what it takes to get you to respond to them in the way that their bio mother did — give them away, reject them, etc. “Roles” in the family are not defined verbally or even understood cortically. They are the automatic place we fall into and feel safe in, even if unhealthy. I have known children who beg their mother to kill them and call themselves evil. These were not necessarily children who were adopted later in life. These can easily be children who were adopted at birth. And they will repeat disturbing behaviors again and again trying to get you to do what they think is right because at some level it is familiar. Rejection is part of their incipient memory.
The prenatal environment is already establishing the role of the child in the world. And of course siblings also play a role. A family is all of its members.
Now imagine the family who wants to put the new adopted baby into the role of a fun little playmate for their only child, or wants to have a baby to love again and expects to get that love back in kind, or who pictures their adopted child going to the same schools, playing the same sports or the same instruments that they did. Disaster might be just around the corner for this family. It is hard to give up ALL expectations to raise a child who comes to you with a history you do not know or don’t understand. But perhaps bringing the dynamics and expectations of the whole family to consciousness will help the whole family, including the adopted child, abandon detrimental and unhealthy roles.