= 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.<Eutherian-like reproductive specializations in a viviparous reptile by Daniel G. Blackburn and others>[PNAS 81, pp. 4860–4863; embryonic dry mass and placental nutrient transfer]{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>.
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