Temurah 31A

ולד טרפה כו׳. למאן דאמר טרפה (אינה) ילדה ⁠— משכחת לה כגון שנטרפה ולבסוף עיברה.

§ The Mishnah teaches that with regard to the offspring of a tereifa, R' Eliezer prohibits it to be sacrificed on the altar, whereas the Rabbis permit it. The Talmud explains: There is a dispute as to whether a tereifa is capable of giving birth. According to the one who says that a tereifa can give birth, you find this dispute between R' Eliezer and the Rabbis in a case where the animal first became a tereifa and then became pregnant.

ובהא פליגי, דרבי אליעזר סבר: זה וזה גורם אסור, ורבנן סברי: זה וזה גורם מותר.

The Talmud elaborates: And this is the matter over which they disagree, as R' Eliezer holds that when both this permitted factor and that prohibited factor cause a certain situation, the resulting item is prohibited. Here too, as the mother of this offspring is prohibited, the offspring is likewise prohibited, despite the fact that its father is permitted. And the Rabbis hold that when both this permitted factor and that prohibited factor cause a situation, the resulting item is permitted. Therefore, the offspring in this case is permitted like its father.

למאן דאמר טריפה אינה חיה, משכחת לה כגון שעיברה ולבסוף נטרפה, ובהא פליגי: דרבי אליעזר סבר עובר ירך אמו הוא, ורבנן סברי עובר לאו ירך אמו הוא.

By contrast, according to the one who says that a tereifa cannot survive, i.e., cannot give birth, you find this dispute in a case where the animal first became pregnant and then became a tereifa. And this is the point over which they disagree, as R' Eliezer holds that a fetus is considered the thigh of its mother, and therefore just as its mother is prohibited to be sacrificed on the altar as a tereifa, the same applies to its offspring. And the Rabbis hold that a fetus is not considered the thigh of its mother, and consequently there is no reason to prohibit it to be sacrificed on the altar.