Doctors measure head circumference in children to rule out disorders that can cause a baby's head to be too big or too small, or to ensure that a child's head circumference continues to grow at a normal rate. Heredity will influence your child's head size to a degree -- some people just have bigger heads than others -- but a deviation from the normal head circumference in your 1-year-old merits investigation.
Growth During the First Year
Clinicians measure head circumference by measuring your child's head at the widest part. When you encircle the head with a tape measure, you arrive at the head circumference. During the first year of life, your baby's head circumference normally grows from between 12.8 to 14.2 inches at birth to an average of 18.6 inches at 1 year, nursing educator and registered nurse Kathleen Theis explains in her textbook, "Growth and Development Through the Lifespan." A newborn's head circumference is slightly less than 1 inch larger than his chest. Between 6 months and 2 years of age, according to MedlinePlus, the two measurements become equal.
It's easy to see, looking at a baby, that his head is usually disproportionately large compared to older children and adults. A baby's head circumference increases dramatically during his first year, normally by around 33 percent, because his brain will have more than doubled in weight, according to Theis. Most brain growth occurs in the first two years of life; a normally increasing head circumference indicates normal brain growth and maturation. Between ages 1 and 2, a child's head circumference increases by just 1 inch as his proportions become more adultlike, Theis reports. He'll be 17 years old before his head circumference reaches maturity, at around 22.4 inches.
Increased Head Circumference
If your baby's head circumference grows faster than normal -- a condition called macrocephaly -- your doctor will also look for a cause. Congenital syndromes such as Canavan syndrome can cause macrocephaly. Excess fluid buildup within the skull leads to hydrocephalus, one of the most common causes of an enlarged head circumference in infants. Hydrocephaly can be evident at birth or develop later as a result of infection or insult to the brain. The excess fluid puts pressure on the brain, pushing it against the skull and causing brain damage, if not corrected. Your baby's doctor might suggest placing a shunt that drains the excess fluid from the skull into the abdomen, where it's harmlessly reabsorbed. Surgery can remove a blockage that's causing fluid buildup.
Decreased Head Circumference
Around 1 in 6,200 to 8,500 children have microcephaly, a head that's significantly smaller than normal, at birth, according to Children's Hospital Cleveland Clinic. Many children with microcephaly have developmental delays; about 15 percent have normal intelligence, according to the Cleveland Clinic. Your child could develop microcephaly after a brain insult such as meningitis, from severe malnutrition or from a condition such as craniosynostosis, where the skull sutures close prematurely. Doctors generally can't treat microcephaly, unless there's a known cause such as severe malnutrition.