If one buys an animal from a non-Jew and it is not known whether it had given birth or had not given birth: Rabbi Ishmael says: that born of a goat in its first year certainly belongs to the priest; after that, it is a questionable case [of a first-born]. That born of a ewe two years old certainly belongs to the priest; after that, it is a questionable case [of a first born]. That born of a cow or a donkey three years old certainly belongs to the priest; after that, it is a questionable case [of a first born]. Rabbi Akiva to him: if an animal were exempted [from the law of the first born] only with the birth of [actual] offspring, it would be as you say. But they said: the sign of offspring in small cattle is a discharge [from the womb]. In large cattle, the after-birth; in a woman, the signs are the fetus and the after-birth. This is the general rule: Whenever it is known that it had given birth, the priest receives nothing. Whenever it had never given birth, it belongs to the priest. If there is a doubt, it shall be eaten blemished by the owners. Rabbi Eliezer ben Jacob says: if a large domestic animal has discharged a clot of blood, it [the clot] shall be buried, and it [the mother] is exempted from the law of the first born.

Rabban Shimon b. Gamaliel says: if one buys a nursing animal from a non-Jew, he need not fear that perhaps the offspring belongs to another [animal]. If he went among his herd and saw animals which had given birth for the first time nursing and animals which had not given birth for the first time nursing, we need not fear that perhaps the offspring of this one came to the other or perhaps the offspring of the other came to this one.

Rabbi Yose ben Meshullam says: one who slaughters the first born, [first] clears a space with the [butcher's] knife on both sides and tears the hair, as long as he does not remove the wool from its place. And similarly one may tear the hair to show the place of the blemish [to a sage].