Placenta

The placenta is an ephemeral (temporary) organ present in female placental vertebrates during gestation (pregnancy), but a placenta has evolved independently also in other animals as well, for instance scorpions and velvet worms.

Filtration and transfer
The placenta receives nutrients, oxygen, antibodies and hormones from the mother's blood and passes out waste. It forms a barrier, the placental barrier, which filters out some substances which could harm the fetus. Many substances are not filtered out, however, including alcohol and some chemicals associated with smoking cigarettes. Several types of viruses, such as Human Cytomegalovirus, may also cross this barrier; this often leads to various degrees of birth defects in the infant.

Metabolic and endocrine activity
In addition to the transfer of gases and nutrients, the placenta also has metabolic and endocrine activity. It produces, amongst other hormones, progesterone, which is important in maintaining the pregnancy; somatomammotropin (also known as placental lactogen), which acts to increase the amount of glucose and lipids in the maternal blood; oestrogen; relaxin, and human chorionic gonadotrophin HCG. This results in increased transfer of these nutrients to the fetus and is also the main cause of the increased blood sugar levels seen in pregnancy.

After delivery
When the fetus is delivered, the placenta is delivered afterwards (and for this reason is often called the afterbirth). After delivery of the placenta the umbilical cord is usually clamped and severed or may be left attached to fall off naturally which is referred to as a Lotus Birth. In most mammalian species, the mother bites through the cord and consumes the placenta, primarily for the benefit of prostaglandin on the uterus after birth. This is known as Placentophagy. The site of the former umbilical cord attachment in the center of the front of the abdomen is known as the umbilicus, navel, or belly-button.

Top: Human placenta shown a few minutes after birth. The side shown faces the baby with the umbilical cord top right. The white fringe surrounding the bottom is the remnants of the amniotic sac. Bottom: A different placenta displays side that connects to the uterine wall..

Species variation
All mammals other than monotremes and (most) marsupials utilize placentas in reproduction, and are known as placental mammals. Also, some species of snakes utilize placentas.

The shape and exchanging surfaces of placental mammals varies according to species.
 * Ruminants have cotyledonary placenta that is really many small placentas where the fetus' cotyledons interface with the dams' caruncle forming a placentome.
 * Carnivores have a zonary placenta.
 * Perissodactyles have a micro-cotyledonary that grossly ressembles diffuse placentas.
 * Primates have discoid placentas.

Pathology

 * Placenta accreta
 * Placenta praevia
 * Placental abruption