Nonplacental

Nonplacental is a term used in biology to describe organisms that do not develop a true placental organ during gestation. In mammals, the designation generally refers to monotremes and most marsupials, which either lack a placenta entirely or possess only a primitive, transient placental connection. The term helps distinguish these groups from placental mammals, which have a more developed chorioallantoic placenta supporting longer gestation.

Monotremes, which include the platypus and echidnas, lay eggs. The developing embryo relies on yolk and secretions from the mother rather than a sustained placental transfer of nutrients. After hatching or emergence, maternal care continues primarily through lactation rather than extended in utero development.

Marsupials, such as kangaroos, opossums, and koalas, have a short gestation period. They give birth to very altricial offspring that complete most of their development outside the womb, typically in a pouch. Their placentation is rudimentary and usually described as choriovitelline rather than the more complex chorioallantoic placenta seen in placental mammals.

In contrast, placental mammals (eutherians) develop a more sophisticated and invasive placenta that enables longer pregnancies and the birth of more developed young. The nonplacental distinction reflects major differences in reproductive strategies and developmental timing across mammalian lineages.