Shifting linear growth during infancy: illustration of genetic factors in growth from fetal life through infancy.

A percentile linear growth chart, from the third to ninety-seven percentiles, was derived from longitudinal data on 90 normal full-term infants of middle class suburban families, and the individual growth curves of each infant was contrasted to the growth chart for the entire group. Individual shifting in growth rate was noted in two thirds of the infants. To better determine the timing and nature of these shifts in linear growth, a search was made for otherwise normal full-term infants who were at or below the tenth percentile for length at birth and who moved up to the fiftieth percentile or better by age 2 years and those who were at or above the ninetieth percentile at birth and moved down to the fiftieth percentile or less by 2 years of age. Those shifting upward had accelerated linear growth soon after birth; they achieved a new "channel" at a mean age of 11.5 months. Those shifting downward did not decelerate until after the first three to six months; they achieved a new channel by the mean age of 13 months. These findings plus the correlation coefficients relating parental size to the length of the infant at birth, one year, and two years of age are compatible with the following statements: Birth length relates predominantly to maternal size whereas by 2 years of age the length correlates best to mean parental height, reflecting the genetic growth factors of both parents. Those infants "catching-up" after birth usually do so in early infancy, whereas those "lagging-down" tend to do so in midinfancy.