White matter volume continues to increase.

Approaching maturity in early adulthood: Grey matter volume in the cortex continues to decrease in early adulthood, leveling out in a person’s 20s.

Studies find that some structures beneath the cortex (such as the amygdala and hippocampus) increase in volume, while others (such as the striatum) are reduced. This process of selective pruning is affected by the environment and helps make adolescence a time of particular susceptibility to outside influence. The number of synapses between neurons in the cortex is scaled back.

A transition period during adolescence: In teen brains, gray matter in the cortex thins considerably.

Brain areas beneath the cortex (such as those in the limbic system) exhibit relatively little change. The areas of the cortex involved in movement and sensation reached maturity earlier than those involved in executive control, such as the prefrontal cortex, which will continue developing into adulthood. The density of synapses in the brain reaches its height and begins to decrease. Developmental peaks in childhood: The brain grows more gradually, with brain size peaking around age 10 for girls and age 14 for boys.

Brain volume continues growing in the second year (an estimated 15 percent).

White matter pathways between spaced-apart regions of the cortex develop. The first years: Brain volume doubles in the first year of life as connections between neurons grow and other developmental processes unfold-including myelination, which boosts signaling speed between neurons.Major long-range signaling pathways made up of white matter begin to form several months into gestation and establish themselves by birth. The brain begins to develop its wrinkled outer surface. Rapid growth in the womb: By the last trimester of pregnancy, a fetus’s brain undergoes major increases in size, folding of the brain’s brain’s outer layer (cortex), and the development of connections between neurons.But there are some major trends in the construction and remodeling that take place: The process of brain development through childhood and into adulthood is far from straightforward, with change happening at different rates in different parts of the brain.

Neurons in the brain