Growth and development refers to the growth of the individual in size as determined by anthropometric measurements of body weight, length, circumference, and weight/length ratio, as well as changes in body composition. This article will focus on growth and development of the fetus, as most of the relevant concepts about growth and development apply to the fetal period of development and this period encompasses the greatest changes in body proportion and composition during the life of the individual. Fetal growth occurs by increases in cell number and size. In the first third of gestation, during the embryonic period, growth occurs primarily by increased cell number (hyperplasia); in the middle third of gestation, cell size also increases (hypertrophy), while the rate of cell division becomes stable. In the last third of gestation, the rate of cell division declines, while cell size continues to increase.
Many terms are used to describe variations in growth. For example, human newborns are classified as having normal birth weight (greater than 2500 g), low birth weight (less than 2500 g), very low birth weight (less than 1500 g), or extremely low birth weight (less than 1000 g). Obviously, classification by weight alone says little about growth rate, as most infants with less than normal birth weights are the result of a shorter than normal gestation, i.e., they are preterm. Similarly, classifying newborns according to duration of gestation (e.g., preterm, term, or post-term) on the basis of birth weight also is erroneous, because infants with intrauterine growth restriction (IUGR) are smaller and macrosomic infants of diabetic mothers are larger than normal at any gestational age. Furthermore, it is inappropriate to label newborns as abnormally grown when their birth weight is less than some arbitrarily determined 'normal' birth weight, but their mother was quite small to begin with; such new-borns are considered constitutionally small but not abnormal.
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