Surrogacy in Halacha
There has been considerable Halakhic literature discussing the Halakhic ramifications of surrogacy, and how it can be done with the greatest sensitivity toward the Halakhic considerations. One of the basic questions is whether the Torah considers maternity based on genetics or by the woman who carried the pregnancy. Obviously, for much of history, there was little distinction, but with modern innovations, this has become a common question. The ramifications of this question are endless; who must the child honor, who would be considered the child's siblings and relatives, and, if one of the women was not Jewish, would this child need to convert?
One of the common sources that are considered to resolve this question involves the story of Leah and Rochel and their pregnancies. Leah and Rochel knew that Yaakov was destined to have 12 sons. When Leah became pregnant with her 7th child, she was concerned that Rochel would only have one child, less than either of Yaakov’s two handmaid turned wives. The Medrash relates that Leah davened, and her pregnancy turned to a girl who she named Dina. In the Midrashic version of the story, it says that Leah's fetus went to Rochel, who turned out to be Yosef, while Rochel, who had been pregnant with a girl, had her fetus switched into Leah. While this would seem to imply that maternity is determined by gestation, there are Rishonim who explain that Shimon, the son of Leah, was able to marry Dina because she was considered Rochel’s daughter (and therefore not his sister).
Ultimately, one must know how to apply a Midrash that relays a miraculous event to practical Halacha
Tzitz Eliezer (15:45, 19:40, 20:49 & 22:55), Nishmas Avraham 4, E.H. 2
Adapted from medicalhalacha.org