Placentation is best known in live-bearing mammals (Theria), but also occurs in some fish, reptiles, amphibians, a diversity of invertebrates, and flowering plants.

In vertebrates, placentas have evolved more than 100 times independently, with the majority of these instances occurring in squamate reptiles.

The placenta can be defined as an organ formed by the sustained apposition or fusion of fetal membranes and parental tissue for physiological exchange.

Monochorionic placentation occurs when monozygotic twins develop with only one placenta and bears a higher risk of complications during pregnancy.

(Primates do not form a definite choriovitelline placenta by fusion, but strong expression conservation suggest that the yolk sac remains useful.

[13] Placentae form following implantation into uterine tissue (as seen in mammals) and formation is likely facilitated by a plasma membrane transformation.

[1][18] Research into the genetic mechanisms that underpin the evolution of the placenta have been conducted in a diversity of animals including reptiles,[19][20] seahorses,[21] and mammals.

For example, in male seahorses the underbelly skin has become highly modified to form a pouch in which embryos can develop.

[19][23] Placental functions include nutrient transport, gas exchange, maternal-fetal communication, and waste removal from the embryo.

In mammals, maternal-fetal communication occurs via the production of a range of signalling molecules and their receptors in the chorioallantoic membrane of the embryo and the endometrium of the mother.

[20] This suggests that maternal fetal communication has evolved by utilising the existing signalling molecules and their receptors, from which placental tissues are derived.