Definition of Normal growth rate

Normal growth rate: The speed with which normal growth occurs in length before birth and in height after birth.

Fetal growth is critical to a person's eventual height. Before birth, the key measure is the crown-rump length -- the distance from the top of the head (the crown) to the buttocks (the rump). The fastest growth rate for a human is during embryonic life. If sustained, it would provide 50-60 cm (close to 2 feet) of growth per year.

This extraordinary rate of growth is largely independent of growth hormone. Growth hormone only begins to play a role in growth in the final weeks before birth. The growth of the embryo and fetus is mainly mediated by maternal nutrition and by growth factors such as fibroblast growth factor, epidermal growth factor, transforming growth factors alpha and beta, insulin, and insulin-like growth factors (IGF-I and IGF-II).

Problems with the production of any of these growth factors or problems with maternal nutrition are associated with intrauterine growth retardation (IUGR). The dominant effect of the intrauterine environment is reflected in the fact that the mid-parental height (the average height of the two parents) has little relation to the length of the baby at birth.

Intrauterine growth restriction (IUGR) is, in turn, associated with an increased risk of health problems later in life, including hypertension, cardiovascular and cerebrovascular disease, insulin resistance, and type 2 diabetes. The reasons are conjectural (rife with guesswork).

The three components of postnatal growth -- infancy, childhood, and puberty -- represent different modes of growth regulation. The growth rate during infancy is rapid but sharply decelerating and is principally dependent on nutrition. Endocrine factors have an increasingly important role from 1 year of age. During the first 2 years, a period of "catch-up" or "catch-down" growth commonly takes place while the infant establishes their own growth trajectory. Thereafter the correlation increases between the mid-parental height and the child's height and between the child's height at any given time and their eventual adult height.

Childhood growth depends largely on endocrine factors such as growth hormone and thyroxine. The average growth velocity by age 6 falls to (2 or so inches (5.0-5·5 cm) per year, with minimum sex differences in the growth rate until the onset of puberty.

The growth spurt at puberty is fueled by the secretion of growth hormone and sex steroids. The timing of this growth spurt is extremely variable. It gives rise, on the average, to a difference in adult height between men and women of about 4-5 inches (14-15 cm).