Placentation

In biology, placentation refers to the formation, type and structure, or arrangement of placentas. The function of placentation is to transfers nutrients from maternal tissue to a growing embryo. Placentation is best known in pregnant female mammals (eutheria), but also occurs in other animals, eggs (yolk sac placentation) and flowering plants.

Placentation in mammals
In placental mammals, the placenta forms after the embryo implants into the wall of the uterus. The developing fetus is connected to it via an umbilical cord. Animal placentas are classified based on the number of tissues separating the maternal from the fetal blood. The placentation types found in animals are:
 * endotheliochorial placentation (e.g. in most carnivores like cats and dogs)
 * epitheliochorial placentation (e.g. in ruminants, horses, whales)
 * hemochorial placentation (e.g. in most primates, including humans)

During pregnancy, placentation is the formation and growth of the placenta inside the uterus. It occurs after the implantation of the embryo into the uterine wall and involves the remodeling of blood vessels in order to supply the needed amount of blood. In humans, placentation takes place 12-18 weeks after fertilization.

In the case of twins, dichorionic placentation refers to the presence of two placentas (in all dizygotic and some monozygotic twins). Monochorionic placentation occurs when monozygotic twins develop with only one placenta and bears a higher risk of complications during pregnancy. Abnormal placentation can lead to an early termination of pregnancy, for example in pre-eclampsia.

Placentation in plants
In flowering plants, placentation occurs where the ovules are attached inside the ovary. The ovules inside a flower's ovary (which later become the seeds inside a fruit) are attached via funiculi, the plant equivalent of an umbilical cord. The part of the ovary where the funiculus attaches is referred to as placenta.

In botany, the term placentation most commonly refers to the arrangement of placentas inside a flower or fruit. Plant placentation types include:
 * Basal placentation: The placenta is at the base (bottom) of the ovary.
 * Apical placentation: The placenta is at the apex (top) of the ovary.
 * Parietal placentation: The placentas are in the ovary wall within a non-sectioned ovary.
 * Axile placentation: The ovary is sectioned by radial spokes with placentas in separate locules.
 * Free central placentation: The placentas are in a central column within a non-sectioned ovary.