Bekhorot 19: Questionable First-borns

2 mishnayot to close out chapter 2! Two ewes bear 2 male lambs -- both are first-born. One male, one female - the kohen gets the male. But with twins, what goes to the kohen? What about a C-section? Chapter 3 - and a new mishnah: When a Jew purchases an animal from a non-Jew... has that animal given birth already? Or no? And the certainty for not having given birth depends on the youth of the animal. A goat is considered certainly off-spring-free until 1 year old; a sheep, until 2 years old; and an ox or donkey until 3 years old. What if the animal conceived and miscarried, instead of bearing offspring?