In humans, growth begins at conception and continues until the late teens or early 20s. Typically, well- nourished humans grow fairly rapidly during the first two trimesters (6 months) of fetal development, but growth slows during the third trimester. After birth, growth rates increase and remain fairly rapid for about 4 years, at which time they decrease again to relatively slow, steady levels, which are maintained until puberty. At puberty, there’s another very pronounced increase in growth. During this so-called adolescent growth spurt, Western teenagers typically grow around 4 inches per year. Following the adolescent growth spurt, the rate of growth declines again and remains slower until adult stature is achieved by the late teens (Fig. 16-3).
Growth curves for boys and girls are significantly different, with the adolescent growth spurt occurring approximately 2 years earlier in girls than in boys. At birth, there’s slight sexual dimorphism in many body measures (for example, height, weight, head circumference, and body fat), but the major divergence in these characteristics doesn’t occur until puberty. The head is a relatively large part of the body at birth. The continued growth of the brain after birth occurs at a rate far greater than that of any other part of the body with the exception of the eyeball. At birth, the human brain is about 25 percent of its adult size. By 6 months of age, the brain has doubled in size, reaching 50 percent of adult size. It reaches 75 percent of adult size at age 2½ years, 90 percent by age 5, and 95 percent by age 10. There’s only a very small spurt in brain growth at adolescence, making the brain an exception to the growth curves characteristic of most other parts of the body. As we’ll see later in this chapter, this pattern of brain growth, including the relatively small amount of growth before birth, is unusual among primates and other mammals. By contrast, the typical picture for mostmammalian species is that at least 50 percent of adult brain size has been attained prior to birth. For humans, however, the narrow pelvis necessary for walking bipedally imposes limits on the size of the neonatal head that can be delivered through it (Rosenberg and Trevathan, 2001). That limitation, in addition to the value of having most brain growth occur in the more stimulating environment outside the womb, has caused human infants to be born with far less of their total adult brain size than most other mammals.