Afterbirth

The placenta is an ephemeral (temporary) organ present only in female placental mammals during gestation (pregnancy).

The placenta is composed of two parts, one of which is genetically and biologically part of the fetus, the other part of the mother. It is implanted in the wall of the uterus, where it receives nutrients and oxygen from the mother's blood and passes out waste. This interface forms a barrier, the placental barrier, which filters out some substances which could harm the fetus. However, many other substances are not filtered out, 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 in various degrees of birth defects in the infant.

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 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.

The placenta is connected to the fetus via the umbilical cord which is composed of blood vessels and connective tissue. 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 in order to avoid predator attraction.

Eating the placenta, called placentophagy, is also done by humans in some cultures.

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.

Non-placental mammals
The only non-placental mammals are the monotremes, which are egg-laying mammals found only in Australia and New Guinea, and marsupials. (Some marsupials have a rudimentary placenta that functions for only a short time; Molly Kalafut's "About Marsupials" points to the bandicoot as the only living example.) According to The Columbia Encyclopedia, marsupials, which are now found primarily in Australia and the surrounding region, have evolved placental analogues in those areas where few native placental mammals arose. In other areas, marsupials were largely displaced by the more efficient reproduction of placental mammals.