Viviparity

Viviparity is reproduction in which embryos develop inside a parent and offspring are born alive. It occurs in many animal groups and has evolved through different reproductive strategies.

Placenta: An organ that develops during pregnancy and mediates exchange between a developing offspring and its parent. In placental viviparity, it supplies embryos with nutrients and oxygen while removing waste.
Embryo: An early developmental stage of an animal after fertilization and before birth or hatching. Viviparity describes where this developing organism grows before birth.
Placental mammals: Mammals whose embryos develop in the uterus and connect to the parent through a placenta. They are the most familiar example of viviparity with extensive maternal-fetal exchange.
Convergent evolution: The independent evolution of similar traits in distinct evolutionary lineages. Viviparity arose independently many times, rather than from one live-bearing ancestor.
Ovoviviparity: A reproductive strategy in which eggs are retained inside a parent until they hatch, with embryos nourished mainly by yolk. It resembles live birth, but embryos generally receive no substantial nourishment from the parent.
Fertilization: The fusion of gametes that initiates development of a new organism. In many viviparous animals, fertilization occurs internally before embryos remain in the parent.
Marsupials: Mammals that give birth to highly underdeveloped young, which usually continue development attached to a teat. Their short gestation and prolonged postnatal development offer a distinct mammalian pattern of live birth.
Reproductive strategy: A set of traits that shapes how an organism produces and invests in offspring. Viviparity is one strategy among varied ways animals reproduce.
Oviparity: Reproduction in which animals lay eggs and embryos develop outside the parent’s body. It contrasts with viviparity in where embryos develop and how offspring emerge.
Amniotic egg: An egg with membranes that protect and support an embryo in a fluid-filled environment. Its membranes also function inside many live-bearing amniotes, even when no shell is laid.

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