Reference Work Entry

Encyclopedia of Child Behavior and Development

pp 1113-1114


  • Jill A. AndersonAffiliated withSchool of Public Health, University of Texas Health Science Center - Houston




The placenta is a semi-permeable membrane that serves as a support organ for the fetus by facilitating the exchange of oxygen, nutrients, and waste products between the fetus and the mother.


The placenta is a composed of a semi-permeable membrane that is dark reddish-blue or maroon in color. It averages nine inches (22 cm) in length and seven inches (18 cm) in diameter. The placenta typically weighs about one pound (500 g).

Relevance to Childhood Development

The placenta develops out of tissue from both the outer layer of the blastocyst (trophoblast) and the lining of the mother’s uterus. It is connected to the fetus via the umbilical cord. The purpose of the placenta is twofold. First, as a semi-permeable membrane, the placenta serves as an exchange site between the fetus and the mother by permitting the passage of some materials between them. Oxygen and nutrients are transmitted from th ...

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