Do Any Mammals Lay Eggs? Scientists recognize three major types of living mammals: placentals, marsupials, and monotremes, all of which produce milk to nourish their young. Placentals and marsupials are viviparous, meaning they give birth to live offspring. Marsupials, such as kangaroos, koalas, and our local Virginia opossum, give birth to very immature, embryo-like offspring that complete their development outside the womb usually attached to a nipple in a pouch. In contrast, placentals, such as dogs, cats, and humans, give birth to more developed offspring and have no pouch.

Source: Carnegie Museum of Natural History