Determining when the human head stops growing is complex because the “head” is composed of two distinct parts that follow different developmental timelines: the neurocranium, which encases the brain, and the viscerocranium, which forms the face. Head circumference growth is primarily driven by the rapid expansion of the brain in early childhood. The skeletal structure of the face, however, continues to remodel and increase in size long after the brain’s growth has stabilized. Understanding the timeline requires separating the early, fast growth of the brain and its casing from the slower, prolonged development of the facial bones.
The Early Developmental Timeline of Brain Volume
The primary factor dictating the rapid increase in head size during infancy and early childhood is the growth of the brain itself. Brain volume increases rapidly in the first few years of life, which is why doctors routinely measure head circumference during this period. The total volume of the brain doubles in the first year after birth and increases by another 15% in the second year, reaching approximately 80% of its adult volume by age two.
The brain continues this rapid expansion through the preschool years, acquiring about 90% of its final adult volume by the age of six. This early growth is not primarily due to the creation of new neurons, which mostly occurs before birth, but rather to processes like neural proliferation and myelination. Myelination involves the formation of a fatty protein sheath around nerve fibers, which increases the speed and efficiency of neural communication.
Myelination contributes to the growth seen in the brain’s white matter and continues into adolescence and early adulthood, particularly in the frontal and parietal lobes. However, the most significant physical expansion of the skull, which accommodates the brain, concludes when this rapid volume increase slows down. While the neural pathways continue to mature, the physical volume that determines the head’s circumference is largely fixed by the time a child enters elementary school.
The Continued Growth of the Cranium and Facial Structure
The bony components of the skull and face continue their growth long after the brain has achieved most of its size, leading to continued changes in head shape. The early growth of the cranium is facilitated by fontanelles (soft spots) and sutures (flexible joints), which allow the skull to expand rapidly to accommodate the growing brain. While the cranial vault plates slow significantly after age seven, the facial skeleton follows a different growth pattern linked to overall body development.
The maxilla (upper jaw) and mandible (lower jaw) undergo significant remodeling and growth during the adolescent pubertal growth spurt. The growth of the mandible is often one of the last skeletal components of the head to reach its full adult size. For females, most growth in the maxilla and mandible is completed around age 15 and 16, respectively.
In males, this skeletal growth continues slightly later, with the maxilla and mandible completing growth around age 18, though growth can occur until age 20 or 21 in some individuals. The nasal structure also undergoes a second growth spurt during puberty, with final development completed by 16 to 18 years in girls and 18 to 20 years in boys. This distinction between the early cessation of brain-driven cranial growth and the later completion of facial bone growth explains why the head’s overall appearance continues to change throughout adolescence.
Key Factors Determining Adult Head Size
The final size of an adult head is determined by a combination of factors, with genetic inheritance playing the predominant role. Genetic influences on head circumference are stable throughout development, suggesting that a person’s final adult head size is largely predicted early in life. Specific genes associated with head and intracranial volume have been identified, demonstrating the strong heritable nature of this trait.
Environmental factors, such as severe malnutrition or chronic illness during early development, can also impact final head size. Conditions that affect brain growth directly, like perinatal asphyxia, can lead to acquired microcephaly (a head circumference smaller than average). Conversely, macrocephaly (a head size larger than average) can result from an underlying condition leading to increased brain tissue or fluid.
Congenital conditions also influence the final structure and size of the head. For example, craniosynostosis involves the premature closure of the cranial sutures, which restricts growth perpendicular to the fused suture and results in an abnormally shaped head. Monitoring head circumference in early life is an important clinical tool, as deviations from normal growth curves can indicate underlying issues with brain development or skull formation.