Placenta

The placenta is a temporary organ that forms in the uterus of placental mammals during pregnancy. It links the developing offspring to the mother's blood supply, allowing exchange of oxygen, nutrients and waste while keeping the two circulations separate.

Branching projections called chorionic villi, which come from the embryo, are bathed in maternal blood, creating a large surface for diffusion. Oxygen and nutrients pass to the fetus, and carbon dioxide and wastes pass back. The placenta also makes hormones, including human chorionic gonadotropin and progesterone, that sustain the pregnancy, and it transfers maternal antibodies to the fetus. At full term it weighs roughly half a kilogram and is delivered after the baby.

The placental barrier blocks many pathogens, but not all. The Zika virus, rubella and the malaria parasite can reach the fetus or harm the placenta. In pregnancy, red cells infected with Plasmodium falciparum bind to a sugar called chondroitin sulfate A in the placenta through a parasite protein, VAR2CSA, which causes placental malaria and low birth weight, a danger described under malaria. Infection with Zika virus in pregnancy can cause birth defects.

The placenta arises from the outer cells of the early embryo, which form during embryogenesis and later after fertilization implant in the uterine wall. Genes of viral origin, derived from ancient retroviruses, were co-opted to help form its cell layers. Marsupials such as the koala have simpler placentas than placental mammals.

• Malaria — a disease with a placental form
• Embryogenesis — the development that forms it
• Zika Virus — a virus that can infect the fetus