Head circumference for girls at 17 months

At 17 months, the median head circumference for girls on the WHO Child Growth Standards is 46.1 cm (18.1 in): half of girls this age measure less and half more. The middle half of girls, from the 25th to the 75th percentile, falls between 45.1 and 47.0 cm, a spread of 1.9 cm. The outer lines of the chart, the 2nd and 98th percentiles, sit at 43.3 and 48.8 cm.

Since 16 months, the median has risen by 0.2 cm in one month. By 18 months it reaches 46.2 cm. That monthly change is less than half the 2.7 cm of the first month, because growth slows steadily through the first two years.

For boys at 17 months the median is 47.2 cm, 1.1 cm more than for girls, 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 17 months is 46.3 cm. A girl 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 girl at 17 months who measures 46.5 cm is at about the 63rd percentile, meaning that out of 100 girls this age, about 63 measure less.