The placenta is an ephemeral (temporary) organ present only in female placental vertebrates during gestation (pregnancy).
All mammals other than monotremes and (most) marsupials utilise placentas in reproduction, and are known as placental mammals.
Also, some species of snakes utilise placentas.
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 to various degrees of birth defects in the infant.
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).