Fetal growth during the last third of gestation requires large increases in nutrient supplies and appropriate utilization of these nutrients. Nutrient substrate supply is coupled with increased development of anabolic hormones and growth factors in fetal tissues and fetal plasma to produce increased nitrogen and carbon deposition in protein, carbohydrate deposition in glycogen, and fatty acid, glycerol and triglyceride deposition in adipose tissue. Growth of these tissues gradually replaces water in the fetal extracellular space.
Chemical composition studies of normal human infants are limited. Based on data from 15 studies that included 207 infants, nonfat dry weight and nitrogen content (predictors of protein content) show a linear relationship with fetal weight and an exponential relationship with gestational age (Figure 3). As gestation proceeds, larger fetuses grow faster than smaller fetuses, and protein accretion follows accordingly.
Fetal water content increases directly with body weight, but not proportionally to body weight, as fetal body water, expressed as a fraction of body weight, decreases with advancing gestation. The relatively large growth of adipose tissue in the human fetus further dilutes the body concentration of water. Extracellular water, as a fraction of fetal body weight, also decreases more than intracellular water as gestation advances; this is mainly due to increasing cell number and increasing cell size, rather than the intracellular concentration of osmotic substances.
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