Post History

Some say the legal mother is the birth mother.  Targum Yonatan says that Dinah, Jacob's daughter, was conceived in Rachel's womb, and Joseph in Leah’s womb, but that God switched the embryos so Rachel could give birth to a son, Joseph. [Targum Yonatan on Genesis 30:21] The justification is that in the Torah, all six times Leah has a son, it says:  “Leah conceived and bore a son”.  But when it comes to Dinah, it just says:  “Leah bore a daughter”. [Gen. 29:32-35; 30:17-21]  Yet, the Torah clearly refers to Leah as Dinah's mother [Gen. 34:1].  So we learn that she who gives birth is the mother, not she who produced the egg. 

But the Talmud may imply that conception, not birth, determines the mother.  It asks:  If one takes a fetus from the womb of one animal, and places it in the womb of another animal, who is the mother?  They discussed this because there are special laws for first-born animals.  The Sages’ answer is Teku, which is equivalent to “We do not know”. [Ḥullin 70a]  But Maimonides rules that the first animal is the mother if the transfer occurs after forty days from conception, implying that conception, not birth, defines the mother, at least after the first forty days. [Rambam, Mishneh Torah, Sefer Korbanot, Hilchot Bekhorot 4:18]

So far, most rabbinical authorities rule that the one giving birth is the mother.  But the debate continues.