Two patterns of care and nurture of the embryo have evolved in animals: oviparity (egg laying) and viviparity (live bearing).

Oviparous animals lay eggs in the environment, and their embryos develop outside the mother’s body. Oviparous terrestrial animals such as insects and reptiles—including birds— protect their eggs from desiccation with waterproof membranes or shells. Oviparity is possible because eggs are stocked with abundant nutrients to supply the needs of the embryo. Some oviparous animals engage in various forms of parental behavior to protect their eggs, but until the eggs hatch, the embryos depend entirely on the nutrients stored in the egg.

Viviparous animals retain the embryo within the mother’s body during its early developmental stages. The embryo is nourished and its wastes removed through exchanges with the mother’s tissues. Viviparity occurs in many vertebrate groups, but with the exception of the prototherians (the monotremes), viviparity is universal in mammals. Therian mammals have a specialized portion of the female reproductive tract, the uterus or womb that contains the embryo until it is born. The early embryo arriving in the uterus from the Fallopian tube implants into the lining of the uterus. Implantation involves interactions between the tissues of the embryo and the uterus to form a placenta. Intimate contact in the placenta between the blood of the embryo and the blood of the mother facilitates the exchange of nutrients and wastes.

In most non-mammalian viviparous animals, such as garter snakes and the well-known aquarium fish the guppy, fertilized eggs are retained in the mother’s body until they hatch. These embryos still receive nutrition from stores in the egg, so this reproductive adaptation is called ovoviviparity.