Physical Growth

Growth differences between males and females begin before birth and continue until adulthood. Generally, boys are larger than girls throughout gestation, so that when they are born at full term (forty weeks), male newborns usually weigh about 150 grams (5.3 ounces) more than females, and are about one centimeter (0.4 inches) longer. Even though they are smaller than their male counterparts, female babies are usually more mature skeletally and neurologically at birth.

After birth, most body dimensions, such as stature, body circumferences, and weight, follow a similar pattern of growth: a period of very rapid growth in infancy, slower growth during middle childhood, a very rapid growth phase or spurt in adolescence, and a period of rapidly decelerating growth, ending with adult size. Obviously, some body dimensions, such as weight or fatness, can continue to change throughout adulthood. The different phases of postnatal growth can be appreciated more easily by looking at the rates of growth, or velocity, in addition to attained size.

On average, boys are taller and heavier than girls at every postnatal age, except from about nine to thirteen years. The reversal of size differences at these ages results from girls entering their adolescent growth spurt about two years earlier than boys. Boys usually end up about nine to thirteen centimeters (three to five inches) taller and seven to nine kilograms (fifteen to twenty pounds) heavier than girls at eighteen years of age. This is primarily because boys grow approximately two years longer than girls do before their spurt, and because the spurt of boys usually is more intense and lasts a little longer than that of girls.