OKay, I want to end this month with more than one post, so I will stop being lazy and post my latest chaburah.
Rashi on the posuk "Shaul ben HaCana'anis" brings the medrash that Shaul was the son of Dina and Shimon. The question is that even a Ben Noach is not allowed to marry his maternal sister. How then could Shimon marry Dina. One approach to answer this question is to use another medrash. The Targum Yonason Ben Uziel tells us that when Leah was pregnant with her 7th child she davened to Hashem that it should be a girl. At the same time, Rochel was also pregnant. Hashem caused the fetus in Leah to be switched with the fetus in Rochel. Rochel gave birth to Yosef and Leah gave birth to Dina. Since Dina was conceived in Rochel's womb you can argue that her mother was really Rochel. Consequently, she was muttar to marry Shimon.
In case you think that this is all derush, there is a very important halachic nafka mina. When a baby is born in-vitro or through a surrogate mother, who is considered the mother -the one who supplies the egg and conceives the child or the one who gives birth to the child. (or in the word of Rabbi Bleich is motherhood defined by parturition or by gestation).
There are a number of teshuvos/articles written on this topic. Rabbi Bleich has an extensive article in his Contemporary Halacha vol 4. Techumim vol 5 also has a discussion on this topic. Finally, there is also an article in the Jouranal of Halacha and Contemporary Society vol 38.
Just to note, we are not discussing whether it is muttar l'chatchila to do such a thing, only b'dieved if it was done who is the mother.
I will just bring 1-2 proofs for each side of the equation.
Parturition or Childbirth
1) The Tzitz Eliezer quotes the sefer Tzur Ya'akov who actually wants to bring a rayah from the Yonason ben Uziel quoted above. However, the rayah is in the opposite direction. Since the torah lists Dina as Leah's daughter and Yosef as Rochel's son, we see that motherhoood is based on teh woman who gives birth to the child.
Rav Shternbach in an article in a medical torah journal called "b'shvilei u'refuah" argues that you can't bring a rayah from derush. Furthermore, there is another pshat, that the fetus' weren't switched but rather the neshamos were switched.
2) The sefer Even Yakar (quoted by the Tzitz Eliezer) discusses a case of an ovarian transplat (which seems to have never happened) and he brings a rayah from a gemara in Sotah 43B. The gemara says that if you graft a tree less than 3 yrs old onto a tree that is older than 3 years, the new tree is patur from orlah. We see you follow the mother who gave birth.
3) Rav Zalman Nechemiah Goldberg in Techumim vol 5 brings a rayah from Yevamos 97b. The gemara discusses a case of a non Jewish lady who converts when she is pregnant and then has twins. The gemara says these twins are considered brothers and there is a chiyuv yibum since at birth the mother was Jewish. Again we see you follow childbirth.
Rabbi Bleich brings a Rabbi Akiva Eiger (also quoted in the Chavtzeles HaSharon on Vayigash) in Yorah Deia (SIman 87) as a proof that we follow conception. The halacha is that milk from a shechted animal can not create an issur basar v'chalav because we need "chaleiv imo"=milk from an animal who can be a mother. A shechted cow can not be a mother. Rabbi Akiva Eiger asks what is the din with a cow that is a treifa. Such a cow can not give birth but it can conceive. He says in such a case the cow is considered a mother. From here we see that motherhood follows conception.
Parturition and Gestation
Rabbi Bleich wants to say that both women are considered the mother. By kilayim of animals, the gemara says "choshishin l'zera ha'av" and the animal is considered to be a species from the father and the mother. Here too, the women who conceived contributed to this child and she should also be considered the mother.
According to the Jouranal of Halacha and Contemporary Society most poskim say the one giving birth to the child is the mother. But as always ask your local Orthodox poseik.