Hello kollel!
The gemara notes that a a pregnant nochri who converts, the child doesn't need tevilah and rejects a question that the mothers body may be a chatziza for the baby in the womb.
Does this mean that if a boy, when he has a bris on the eighth day, it's a bris of geirus and not the bris of a born Jew? Bc if he is simply a full Jew by virtue of being born to a now Jewish mother there should be no question about chatzitza at all.
If he's treated as a ger does this idea resolve the question from several daf prior about whether milah or tevila is primary for geirus?
And going back to a prior idea, does the stage of the pregnancy have any ramifications (Ie the mother converts with a 20 day old embryo bs converting the day before she gives birth)?
1) Yes, the Teshuvas Binyan Tziyon 1:22 (by the author of the Aruch la'Ner) makes this Diyuk -- that the fact that the Gemara states that the Tevilah of the mother helps for the baby proves that the baby is a Ger and, therefore, if he was born on a Shabbos the Bris may not be done the following Shabbos.
2) Tosfos above (47b, DH Matbilin) writes that it seems that Milah should come before Tevilah, but then he asks from our Gemara where we see that the Tevilah comes before the Milah. Tosfos answers that our Gemara is different because here it would be impossible to do the Milah earlier on. Therefore, according to Tosfos, one may say that the baby is treated as a Ger, and, generally speaking, Milah should come first, but here there is no choice.
3) I found, bs'd, in Sefer ha'Bris (Perutinski), page 382, Likutei Halachos in the name of Os Bris, Hilchos Gerus #8, that he proves from Nazir 51b that even according to the opinion that an embryo is not considered to be an extension of ("like the thigh of") its mother, that applies only if she is more than forty days pregnant; within the first forty days, the fetus is a part of the mother since the baby has no form of its own.
4)
a) There is a dissenting opinion -- that of Rav Dovid zt"l of Karlin, who holds that one may do the Bris on Shabbos if Shabbos falls on the eighth day, because the baby is not considered to have been born as a Ger. This opinion is cited by Rav Chaim Ozer Grodzinsky zt'l in Achi'ezer 4:44. The baby is not considered to have done Gerus on his own, but rather he is born as a Ben Yisrael who has no family connection to his father.
b) A proof for this is the Gemara below (97b, three lines from the bottom) that says that in the case of twin brothers who were conceived before their mother did Gerus but born after the Gerus (this is the scenario in 78a where the mother went to the Mikvah for Gerus during pregnancy), if later on either of them has relations with his twin brother's wife, he is liable for Kares. This proves that the twin brothers are full brothers mid'Oraisa. We learn that they are born as full sons of the same mother, which means that the Bris on the eighth day does not make them Jewish, since they were fully Jewish at birth. Therefore, the Bris may be performed on Shabbos.
c) Rav Dovid Karliner also cites Kesuvos 11a (and Rashi, four lines from the top of the page) that if a mother brings her child to Beis Din with the purpose of conversion (because he was not born Jewish) when he grows up he is entitled to protest and say he does not want to remain Jewish. In our Gemara (78a), where the mother did Gerus during pregnancy, there is no mention that the baby could refuse later on to remain Jewish. This suggests that he was not born as a Ger.
d) Rav Dovid also cites Bechoros 46a, that if a mother did Gerus while pregnant, the baby needs a Pidyon ha'Ben if the baby is a male and her first child. If he was born as a Ger, it is not possible that he would require Pidyon ha'Ben.
The Achi'ezer discusses this matter further and cites sources which suggest that the baby is considered to have been born as a Ger, but at the end he writes that -- Halachah l'Ma'aseh -- one may rely on Rav Dovid Karliner and perform the Bris on Shabbos.
Dovid Bloom
Kol Tuv,
Dovid Bloom