The placenta exerts strong control over fetal growth by providing nutrients directly or in metabolically altered form and amount. Naturally and experimentally, placental growth precedes fetal growth, and failure of placental growth is directly associated with decreased fetal growth. There is considerable variation in this control. For example, experiments in sheep that limited placental growth did not result in proportionately reduced fetal weight, indicating that either the capacity of the smaller placenta to transport nutrients to the fetus increased adaptively, or that the fetus developed increased capacity to extract nutrients from the placenta and direct those nutrients to growth. More characteristically, though, limitation in placental function to transfer nutrients to the fetus directly limits fetal growth. In fact, fetal growth retardation is seen as a natural and repro-ductively successful (though not perfect) adaptation to nutrient limitation. There is a direct relationship between fetal weight and placental weight in humans, indicating that placental size and fetal size are directly interrelated, although functional interrelationships between placenta and fetus also are important to fetal growth and development.
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