viviparity

Viviparity is a reproductive strategy in which the embryo develops inside the parent and is nourished directly by the parent, rather than from yolk in an externally laid egg. It contrasts with oviparity, in which eggs are laid and develop outside the body, and with ovoviviparity, where eggs are retained inside the mother but are nourished largely by yolk rather than a persistent placental connection. In viviparous species, nourishment is typically provided through a placenta or placenta-like structure, and development occurs within the mother's body until birth. After birth, maternal care such as lactation in mammals often continues to support the young.

Most mammals are viviparous. Placental mammals (eutherians) obtain nutrition through a complex placenta that supports lengthy in utero development. Marsupials are also viviparous, but give birth after a relatively short gestation and complete much of newborn development outside the womb, usually in a pouch, with extensive lactation. Monotremes, such as platypus and echidna, are oviparous and lay eggs.

In other animal groups, viviparity has evolved independently in several reptiles (some snakes and lizards) and in various fishes (including some sharks and rays and some live-bearing bony fishes).

Advantages of viviparity include enhanced offspring protection and potential for extended parental care, which can improve survival in harsh or variable environments. The main costs are high energetic demands on the mother, reduced brood size per reproductive cycle, and potential constraints on rapid geographic spread.