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The placenta is a disk-shaped organ that serves as the interface for maternal and fetal exchange of materials. It is formed early in pregnancy when the outer cell layer that envelops the developing embryo, the chorion, fuses with the uterine wall forming fingerlike projections called chorionic villi. Each villus, surrounded by a pool of maternal blood, contains a network of fetal capillaries through which nutrients and waste products are transferred (although there is no actual exchange of blood). Products harmful to fetal development, such as nicotine, cocaine, alcohol, some medications, and environmental pollutants, may also be transferred to the fetus. Upon completion of the pregnancy, the placenta is expelled from the uterus. For the purposes of diagnosis in cases of complications, physicians may examine the placenta. Complications involving the placenta itself include postpartum hemorrhage, placenta previa, preeclampsia, and intrauterine growth restriction.

Jean Piaget's fascination with children's reasoning began with his work on early Intelligence Quotient (IQ) tests. (AP/Wide World Photos)


Begley, David J., Anthony Firth, and Robin Hoult. Human Reproduction and Developmental Biology. New York: Macmillan, 1980.

Faber, J. J., and Kent Thornburg. Placental Physiology: Structure and Function of Fetomaternal Exchange. New York: Raven Press, 1983.

Jansson, Thomas, and Theresa Powell. "Placental Nutrient Transfer and Fetal Growth." Nutrition 16 (7/8):500-502.

Vander, Arthur, Jane Sherman, and Dorothy Luciano. Human Physiology: The Mechanisms of Body Function, 5th edition. New York: McGraw-Hill, 1990.

Patricia Crane Ellerson

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