 Viviparity

Viviparity is reproduction in which offspring develop within the maternal body and are delivered alive. It contrasts with oviposition, in which an egg is laid before development is completed. Viviparity describes retention and birth; it does not by itself specify how much nourishment is supplied during development.

Some live-bearing animals rely largely on reserves packaged in the ovum, while others receive substantial additional maternal nutrients. A study of the lizard Mabuya heathi compared egg size, fetal growth and placenta structure and found extensive maternal contribution to the developing offspring.^[ref] The comparison illustrates why live birth and the mode of nourishment are separate biological characters, and why a placenta is not exclusive to mammals.