Placental nutrient transfer capacity increases over gestation by increased placental growth, primarily of membrane surface area, allowing for the increase in nutrient supply required for the growing fetus. Glucose is the primary energy substrate for the mammalian fetus and placenta. The fetus metabolizes glucose in several ways, including oxidation for energy requirements and as a carbon source for production of various macromolecules, such as glycogen, glycolytic products, proteins, and fatty acids. Total nitrogen concentration measurements have been used to estimate the rate of protein accretion in tissues, because most of the total nitrogen is represented by amino acid nitrogen uptake. The net uptake of amino acids by umbilical circulation through the placenta represents the dietary supply of amino acids for fetal growth and protein metabolism. The transport of fatty acids and other lipid substances across the placenta and the deposition of lipids in fetal adipose tissue are primarily late-gestation phenomena.