Bechorot 22 - Buying an Animal from a Jew
If one buys an animal from a Jew, then, unlike a non-Jew, this seller knows about various implications of the law of firstborn, and therefore we treat his statements or his silence differently. Rav said that the first offspring of the bought animal is a definite firstborn. Had the animal previously given birth, the seller would praise it now, because subsequent births are far less risky than the first one. He is silent because the animal has not given birth yet.
Shmuel says that the animal only might be a firstborn: the seller assumes that the buyer wants the animal for immediate slaughter, and firstborn simply doesn't matter. Rabbi Yochanan says that the first offspring is definitely not a firstborn: the Jew knows how serious the law of firstborn is, and he would certainly alert the buyer if the possibility of firstborn existed.
What is a sign of miscarriage in a large animal? If it discharged a bloody substance, which looks like a hard cake, it may contains disintegrated remains of a firstborn fetus. It must be buried, because a firstborn is prohibited for benefit. Afterwards, the mother is free from the law of firstborn.
Art: Giovanni Segantini - Goat With Offspring