I have also given a number of shiurim on this subject. You will find the
latest Teshuvah on this subject in Techumim vol. 16, by Rav Ariel of Ramat
Gan. On page 172, you will see the view of Rav Zalman Nechemiah Goldberg. In
his view the mother is the woman who delivers the baby. He expressed this
view some 10 years ago, but as far as I have been able to establish, he has
not changed his mind. Rav Ariel himself thinks that motherhood goes to the
lady whose eggs have been fertized. Recently I asked an eminent rabbi in
Yerushalayim who was close to Rabbi Shlomo Zalman Auerbach and he told me
that the latter was of the opinion that the mother is the woman who gave the
egg. See, however, in that article page 178, that in Sefer Nishmat Adam, it
is reported that Rav Shlomo Zalman regarded motherhood in this case as
doubtful and therefore thought that the baby had two mothers. Therefore,the
Chumrot of both mothers must be applied to the baby. He says that both women
have maternal feelings towards the baby. I feel, however,that this assumption
is questionable in cases where the egg is fertilized in vitro and the host
mother is only doing a favour to a childless couple who cannot have a baby
in a natural way.
I would also like to know more on the subject and further opinions on this
most important question.
Rabbi Ya'akov Grunewald
Pinner Synagogue, London.