Gemara

Question #1 (R. Yirmeyah): Is a fetus in the womb Galgalin (considered a foreign matter) to its mother (to inhibit Tum'ah from her Rekev when she rots)?

Since Ubar Yerech Imo (a fetus is considered like a limb of its mother), it is like part of her body, and it is not Galgalin. Or, perhaps since it was destined to leave, it is considered a separate matter!

Question #2: If a fetus is considered separate, is semen in the womb Galgalin?

Chulin 71b (Mishnah): If a midwife touched a dead fetus in the womb, she is Teme'ah (Mes) for seven days. The mother is Tehorah until the fetus comes out.

(Rabah): Just like Tum'ah Belu'ah (enveloped) does not Metamei, also Taharah Belu'ah does not become Tamei.

Yevamos 67a (Beraisa - R. Yosi): If a Kohen died and left sons and a pregnant wife, her Melug slaves eat Terumah like she does. Her Tzon Barzel slaves do not eat, because the fetus owns a share. A fetus forbids eating, but does not permit;

R. Shimon says, both types of slaves eat.

The Tana'im argue about whether or not we are concerned for the minority (that the fetus is a viable male. Half of all fetuses are male, and some are Nefalim.)

78a (Rava): If a pregnant Nochris converted, her child need not immerse. (Her Tevilah counts for him. She is not a Chatzitzah, for he grows in her.)

Bava Basra 142b (Mar brei d'Rav Yosef citing Rava): A son born posthumously does not decrease the extra share of the firstborn - "V'Yaldu Lo" (we consider only sons who were already born when the father died).

Rishonim

Rambam (Hilchos Tum'as Mes 25:11): If a woman gave birth to a dead baby and then to a live baby, the live baby is Tahor, for he did not touch the dead one after leaving the womb.

Rebuttal (Ra'avad): The mother becomes Tamei Shivah (for seven days) from the first, and she is Metamei the second Tum'as Erev (until nightfall). Also, the second gets Tum'as Erev through touching the mother.

Defense #1 (Tosfos Yom Tov Ohalos 7:5 DH ha'Rishon): The Rambam means that the live baby is Tahor from Tum'as Shiv'ah.

Defense #2 (Minchas Chinuch 363:3 DH u'Mah): Perhaps normally Ubar Yerech Imo, and a fetus is Mekabel Tum'ah with his mother, but once the womb opened (for the birth of the first baby) the second fetus is considered a separate entity, and it is not Mekabel Tum'ah because it is Belu'ah!

Rashash (Ohalos 7:5): The Rambam explains that the first baby was not born in a house.

Rosh (Bava Kama 5:2): Ubar Yerech Imo in every respect, except for Tereifah. (If a pregnant animal became Tereifah, the fetus does not become Tereifah with it.)

Poskim

Shulchan Aruch (343:1): One may not feed an Isur to a minor.

Shach (371:1): The Roke'ach permits a pregnant Kohen's wife to enter Ohel ha'Mes, due to a Sefek-Sefeka (two doubts): perhaps the baby is a Nefel (stillborn), and even if it is viable, perhaps it is a girl.

Question (Magen Avraham 2): In any case she is permitted, because what is Belu'ah (enveloped) does not become Tamei!

Answer #1 (Radvaz 1:200): The Roke'ach permits even if she is about to give birth, and the baby is prone to stick out its head, which is birth.

Keneses ha'Gedolah (YD Reish Siman 371, brought in Birkei Yosef 4): In some places, Kohanim's wives avoid Tum'ah during pregnancy. This is a boorish custom. A fetus cannot become Tamei! R. Azaryah says that they should not enter Ohel ha'Mes, since it is permitted only due to Sefek-Sefeka. This is groundless. The Roke'ach permitted due to the Sefek-Sefeka!

Answer #2 (Birkei Yosef 4): Some say that Tum'ah applies to a fetus, because Ubar Yerech Imo. R. Azaryah derived this from the Roke'ach, who permitted only due to the Sefek-Sefeka. Therefore it is proper to be stringent!

Chasam Sofer (YD 354): If Ubar Yerech Imo, a fetus in a Zarah (non-Kohen) is a Zar! And even if she is a Bas Kohen, if Ubar Yerech Imo, her fetus is considered to be a female, like her (and is not commanded about Tum'ah)! Not everything that is Metamei Kohanim is forbidden to them. Chachamim forbade Kohanim to enter Nochri houses, so one may not bring a Kohen's son there, However, even if something Belu'ah becomes Tamei, since it applies only to a fetus in its mother, Chachamim did not decree to forbid regarding Kohanim.

Question (Binyan Tziyon 96): Even if Ubar Yerech Imo, a male fetus is considered to be male! Tosfos and the Nimukei Yosef asked, since Tevilah before Milah is invalid, why does Rava say that Tevilah of a convert helps for a male fetus? They assumed that he holds that Ubar Yerech Imo. If the fetus were considered a female, this would be no question!

Answer (Binyan Tziyon): If Ubar Yerech Imo, it is contradictory to say that the fetus is male and the mother is female! The Rashba and Ritva say that Tosfos asked according to the version that Rava holds that Ubar Lav Yerech Imo. The fetus is considered a female while inside, but after birth he is a male, and retroactively he was Tamei. Rava needed a verse to teach that a son born posthumously does not decrease the extra share of the firstborn. If a fetus were considered a female, no verse would be needed. At the time of death the fetus did not decrease the firstborn's share. Once the firstborn acquired, he does not lose it when the fetus is born!

Har Tzvi (YD 281): Even though the fetus is not considered a Kohen now, since if his mother enters Ohel ha'Mes he becomes Tamei and will be Tamei when he is born, it is as if she is Metamei a Kohen.

Minchas Chinuch (ibid., DH Im): A fetus is like a limb of the mother. It is not a Kohen!

Question (Binyan Tziyon 97): All Tana'im agree that if we knew that the fetus was male, he would forbid his mother's slaves to eat Terumah even when there are other sons (Yevamos 67). Surely, he is considered a Zar because Ubar Lav Yerech Imo, yet he owns a portion, for he is considered a male!

Answer (Binyan Tziyon): There was a Hava Amina (Bava Basra 142) that a fetus does not inherit. After it is born, how does he take from the other's portion? And even though we hold that a fetus inherits, if the father died right after Bi'ah, surely it is not called a fetus before conception, and possibly until 40 days! We must say that this is due to a Gezeras ha'Kasuv "Ein Lo" - Ayen Alav (see if the Mes will have heirs). One might have thought that this applies even to the firstborn's share.

Avnei Milu'im (82:1): A mother is not a Chatzitzah for a fetus living inside her, regarding Tevilah or Tum'as Ohel. Something Belu'ah is not Metamei or Mekabel Tum'ah if it does not live inside, e.g. a dead fetus or a ring.

Har Tzvi (YD 281): The law of Belu'ah is a Gezeras ha'Kasuv. It does not depend on Chatzitzah. A Tamei ring is not Metamei a Tahor ring (if both were swallowed), even if they touch. (Haga'ah - another proof is that Belu'ah applies inside people, even though people are Mekabel Tum'ah, and cannot be a Chatzitzah.) Perhaps the Avnei Milu'im means that since the mother is not a Chatzitzah, it is as if the fetus is outside, and not Belu'ah.