75 150 225

Gestational age (days)

Figure 3 Nonfat dry weight (A) and nitrogen content (B) are plotted against gestational age for LGA (■, —), AGA (O, —), and SGA (♦,---) infants. (Reproduced with permission from

Sparks JW (1992) Intrauterine growth and nutrition. In: Polin RA and Fox WW (eds.) Fetal and Neonatal Physiology, p. 184. Philadelphia: W.B. Saunders.)

Nonfat dry weight

Comparative aspects of fetal chemical and physical growth in six species are summarized in Table 1. Despite growth rate variations up to 20-fold and weight-specific fat content variances at term up to 16-fold among these species, nonfat dry weight and protein weight-specific contents (as percentages of total weight at term) are constant. Protein concentration is about 12% in all species at term and fetal protein content is linearly related to fetal weight; thus, protein accretion in the fetal rat occurs about 23 times as fast as it does in the human. These species-related differences in growth rate are remarkable and require marked differences in the placental capacity to supply nutrients to the fetus.

Table 1 Growth characteristics and chemical composition at term of selected mammals and a representative human fetus
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