Head circumference for boys at 3 months

At 3 months, the median head circumference for boys on the WHO Child Growth Standards is 40.5 cm (16.0 in): half of boys this age measure less and half more. The middle half of boys, from the 25th to the 75th percentile, falls between 39.7 and 41.3 cm, a spread of 1.6 cm. The outer lines of the chart, the 2nd and 98th percentiles, sit at 38.1 and 42.9 cm.

Since 2 months, the median has risen by 1.4 cm in one month. By 4 months it reaches 41.6 cm. That monthly change is less than half the 2.8 cm of the first month, because growth slows steadily through the first two years.

For girls at 3 months the median is 39.5 cm, 1.0 cm less than for boys, which is why the charts are drawn separately by sex.

On the older CDC 2000 infant charts, which some clinics and websites still use before age 2, the median at 3 months is 41.2 cm. A boy exactly on the WHO median would read as about the 32nd percentile there, one reason two charts can give different answers for the same head circumference.

Reading the table: a boy at 3 months who measures 41.0 cm is at about the 66th percentile, meaning that out of 100 boys this age, about 66 measure less.