Abnormality of skull size Postnatal deceleration of headgrowth Slowing of the rate of head circumference growth occurring after birth, in an individual with normal head circumference at birth, leading to progressive deviation from normal growth curves and, in many cases, acquired (postnatal, secondary) microcephaly. The human brain undergoes dramatic changes in size and connectivity after birth. From birth to age six years, the brain increases in size by four-fold, reaching 90 percent of adult volume. During normal development, the growth of head circumference is largely driven by growth of underlying brain tissue.