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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 from the mother's blood and passes out waste. This interface forms a barrier, the placental barrier, which filters out many substances which could harm the fetus. However, many other substances are NOT filtered out, including alcohol. Any time that a pregnant mother drinks, so does the fetus. Most viruses also easily cross this barrier.

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, becoming an infant, the placenta is delivered afterwards (and is often called the after-birth), and the umbilical cord must then be severed. In most mammalian species, the mother bites the cord and consumes the placenta.

The site of the former umbilical cord attachment in the center of the front of the abdomen is known as the umbilicus, or navel, or belly-button.

The only non-placental mammals are the monotremes, which are primitive egg-laying mammals only found in Australia and New Guinea, and the marsupials, which are found primarily in Australia.

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