Head circumference for boys at 21 months

At 21 months, the median head circumference for boys on the WHO Child Growth Standards is 47.8 cm (18.8 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 46.9 and 48.7 cm, a spread of 1.8 cm. The outer lines of the chart, the 2nd and 98th percentiles, sit at 45.2 and 50.5 cm.

Since 20 months, the median has risen by 0.1 cm in one month. By 22 months it reaches 48.0 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 21 months the median is 46.7 cm, 1.1 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 21 months is 48.3 cm. A boy exactly on the WHO median would read as about the 38th percentile there, one reason two charts can give different answers for the same head circumference.

Reading the table: a boy at 21 months who measures 48.5 cm is at about the 69th percentile, meaning that out of 100 boys this age, about 69 measure less.