Ben Pekuah

If one slaughtered an animal and found in it an eight months’ fetus, either living or dead, or a dead nine months fetus, he need only tear it open and take out the blood. If he found in it a living nine months’ fetus it must be slaughtered, and he would thereby [possibly] incur the penalty for “it and its young,” the words of Rabbi Meir. But the sages say: the slaughtering of its mother renders it permitted. Rabbi Shimon Shezuri says: even if it is eights years old and is plowing the field, the slaughtering of its mother renders it permitted.

One who slaughters an animal and finds it Kosher, then found inside a fetus of 8 (months), whether alive or dead, or a fetus of 9 (months) dead, it may be eaten and does not require slaughter. And if he finds inside a fetus of 9 (months) that is a alive: if it stood on the ground, it requires slaughter, but other treifot do not prohibit it. If it did not place its hooves on the ground it does not require slaughter. And if its hooves are fused even though it stood on the ground, it does not require slaughter. And some are in doubt.

Because an 8 month fetus that lives is as if it is dead since it cannot live.

(2) The Taz writes that there is also no issue of forbidding eating the limb of a live animal. Also there is no issue of slaughtering it and it's mother [on the same day] and it is permissible to slaughter it on the same day as its mother and father.

If it walked on the ground it requires slaughter: From a rabbinic enactment since people may mistake it for other regular animals when they see it being eaten without slaughter, as is explained in the Talmud and the poskim.

(2) One who slaughters an animal and finds an eight or nine month fetus whether it is alive or dead, its helev and sciatic nerve are permitted. A nine month [fetus] is specifically only permitted if it hasn't walked on the ground. However, if it walked on the ground, it is forbidden. There are those who say that if it completed its gestational period and was found alive, even though it did not walk on the ground, its helev and sciatic nerve are forbidden and [consumption] is punished by karet and we remove [from the animal] all the sinews and membranes that are forbidden in all animals.

(7) We are not accustomed to doing this [removing the forbidden fats and sciatic nerve] on a fetus that does not require shechita. There are those who say we should do it if it completed gestation and was found alive. Rema: our custom is to be strict like the second opinion.

We are not accustomed to doing this with a fetus: The explanation is that the slaughter of the mother that the fetus was found in permits the fetus and even its sciatic nerve is allowed.

If a living 9-month fetus was found in the womb of a kosher-slaughtered animal, and the fetus grew up and mated with a regularly-born animal and begot a child, the child cannot be made kosher by shechitah. If the fetus grew up and mated with a female animal who was similarly a fetus from a kosher slaughter, then their children, and grandchildren, for all generations, are also considered ben pekuah. They all require shechita by rabbinic law. However, they are not rendered non-kosher if they have a treifah.

[13th century]

Rabbi Shemarya heard from his rabbi, Rabbi Eliezer son of Rabbi Meshulum son of rabbi Sherira Gaon father of Rabbi Hai Gaon two thing. He allowed, lechatchila, boiling wine and drinking it with idol worshipers. Also he had a ben pekuah and he didn't slaughter it, rather he killed it with a hatchet between the horns and served it at a wedding.

(commenting on YD 13:4)

Since they walked on the ground

Since it walked on the ground (Shach). If it didn't walk, it is permitted by the slaughter of its mother rabbinically. If its mother died, and [the fetus] is a nine month fetus that died, it is forbidden rabbinically. If a nine month fetus lives, it is permitted rabbinically through its own slaughter.

Has no rectification - The fetus is like [an animal] that has only one sign from its mother's side and the second sign is [already] cut. It is alive and dead, and slaughter is not done on that. An animal with one sign and cannot be made kosher....

The issue is that Avraham upheld the entire Torah and therefore it seems that the cow [he served to the angels] was a ben pekuah and shechita permitted it and there was no issue of meat and milk since milk found in a slaughtered cow is permitted [to eat with meat]. And see the Sha'ar HaMelech.

When Hashem forbade milk and meat [together] each on its own was permitted. We do not find that it was established regarding milk of a ben pekuah since the slaughter of its mother permitted it and it is like it is slaughtered. Milk found in the stomach of a slaughtered animal is not forbidden with respect to meat and milk as is known.

We learn that if one who cooks milk found in a slaughtered animal [with meat] there is no issue of milk and meat, so to one who cooks a ben pekuah in milk has no issue of meat and milk since it is considered slaughtered. This is a stretch. Firstly, we don't find in any of the poskim or rishonim or achronim that mention this innovation that the milk of a bat pekuah is not included in the prohibition of milk and meat from the Torah.

One who cooks a fetus in milk is liable, as is one who eats it. However, one who cooks a placenta, skin, sinews, bones, or the inside of horns and talons in milk is exempt. Likewise one who eats it is exempt.