Here’s information from the EWTN web site regarding this matter:
If the mother of an embryo cannot be located or should she refuse
the transfer, certain authors, among whom are some Catholics, have considered the possibility of transferring the embryos into another woman. This would be a case of “prenatal adoption” to be distinguished from surrogate motherhood and heterologous fertilization with a donor oocyte.
In this case there would be no offense to matrimonial unity, nor to the equilibrium of familial relationships, because the embryo would have, from the genetic
standpoint, the same relationship to both adoptive parents. The stronger and more profound bonding which would occur between a child adopted before birth and his adoptive parents ought to lessen the psychological difficulties which at times are seen in traditional adoptions. Moreover, such a solution would highlight the significance of adoption as an expression of the fecundity of marital love and as fruit of a generous openness to life which leads spouses to welcome into their family children whose parents have died or who have been abandoned (, nn. 14, 41; , n. 93), above all in those cases where it is a question of children abandoned because of disabilities or illnesses (, n. 63).
Here’s the whole document:
www.ewtn.com/library/PROLIFE/FROZEMBR.txt