Head circumference for girls at 13 months

At 13 months, the median head circumference for girls on the WHO Child Growth Standards is 45.2 cm (17.8 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 44.3 and 46.1 cm, a spread of 1.8 cm. The outer lines of the chart, the 2nd and 98th percentiles, sit at 42.4 and 47.9 cm.

Since 12 months, the median has risen by 0.3 cm in one month. By 14 months it reaches 45.4 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 13 months the median is 46.3 cm, 1.2 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 13 months is 45.3 cm. A girl exactly on the WHO median would read as about the 45th percentile there, one reason two charts can give different answers for the same head circumference.

Reading the table: a girl at 13 months who measures 45.5 cm is at about the 59th percentile, meaning that out of 100 girls this age, about 59 measure less.