Head circumference for boys at 9 months

At 9 months, the median head circumference for boys on the WHO Child Growth Standards is 45.0 cm (17.7 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 44.2 and 45.8 cm, a spread of 1.7 cm. The outer lines of the chart, the 2nd and 98th percentiles, sit at 42.5 and 47.5 cm.

Since 8 months, the median has risen by 0.5 cm in one month. By 10 months it reaches 45.4 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 9 months the median is 43.8 cm, 1.2 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 9 months is 45.3 cm. A boy exactly on the WHO median would read as about the 42nd percentile there, one reason two charts can give different answers for the same head circumference.

Reading the table: a boy at 9 months who measures 45.5 cm is at about the 65th percentile, meaning that out of 100 boys this age, about 65 measure less.