In his review it is clear that the prime mother is the mother who gives
birth to the child. There is however an aspect of motherhood due to the
woman who contributed the egg likely relevant to whom the resultant
child could marry. However, it seems that eg. the definition of jewishness
would be from the woman who gives birth. Rabbi Moshe Tendler also
provided me this opinion on personal communication.
Furthermore, this is very different than the definition of fatherhood which
clearly comes from the provider of the sperm.
It seems the process of motherhood is complete and defined at the birth of
the child where as fatherhood is defined at conception. Rabbi Bleich's
review supplies a fine scholarly review of the subject with references to
primary halachic and Talmudic sources.
Truly
Paul Claman MD
Director, IVF-ET Clinic
University of Ottawa, Canada