What head circumference represents
In the first two years the brain grows extraordinarily fast, from about 25 percent of adult size at birth to almost 80 percent by age two. Because an infant’s skull bones are still pliable and the fontanelle is open, head circumference follows that growth. That is why doctors measure it at every visit until 24 months, and why a head-circumference-for-age chart sits beside weight and length.
Typical numbers
Average head circumference at birth is about 34 to 35 cm, increasing roughly 2 cm a month in the first three months, then slowing to about 1 cm a month until six months and 0.5 cm a month after that. At one year the average is around 46 cm.
As with the other indicators, values between -2 and +2 are considered normal. Smaller than -2 is called microcephaly, larger than +2 macrocephaly, but both are often simply family traits: a child of large-headed parents may naturally track along the upper line.
Measuring correctly
Use a non-stretch tape. Pass it around the most prominent point at the back of the head and just above the eyebrows, pulling snugly enough to compress the hair. Measure three times and record the largest. Log it in centimetres with one decimal.
When to have it checked
What doctors watch for is a change of track: a head that was on the median and then jumps to +2 within two months, or one that stops growing. If that happens, or if the fontanelle looks bulging and tense while the baby is calm, consult a paediatrician.
This article is general education and does not replace a doctor’s examination. When in doubt, consult a paediatrician.
