This Mishnah will talk about an animal that had a difficult labor, but was giving birth to a Bechor, a firstborn male, which would become sacred at the time of birth. The firstborn of a cow, sheep, or goat is automatically sacred, is brought as a Korban (offering), and is eaten by the Kohanim (priests). If it dies before it is offered, then you’re not allowed to benefit from it. But it only takes effect at birth. Like the Pasuk (verse) tells us, “that will be born.” So, before the Bechor is born, a person is allowed to cut off its limbs and feed them to his dogs.
Mishnah says: If an animal giving birth to a Bechor, a firstborn male, is having difficulty in labor, different limbs are coming out first. It’s not coming out the normal way. A person is allowed to cut off the fetus limb by limb as the limbs come out and feed them to the dogs. As long as the majority of the fetus or the head has not come out of the animal’s body yet, the fetus is not considered to be born and therefore isn’t sacred as a Bechor. But if the majority of the fetus came out before being cut off, it has to be buried. None of it could be fed to the dogs because once the majority comes out, the fetus is considered sacred, has the status of a Bechor, and therefore, once it dies, you’re not allowed to benefit from it whatsoever.
In both cases—whether the fetus was cut limb by limb or the majority of it came out before it was cut off—the animal was dead, but the mother has had her Bechor already. In other words, if she has another baby, then that animal is not a Bechor, because a Bechor is only a Peter Rechem, whatever opens up the womb. This dead fetus opened up the womb, and therefore any further babies that are born are not going to be considered Peter Rechem, and therefore they don’t have the status of a Bechor.