Length for boys at 13 months

At 13 months, the median length for boys on the WHO Child Growth Standards is 76.9 cm (30.3 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 75.3 and 78.6 cm, a spread of 3.3 cm. The outer lines of the chart, the 2nd and 98th percentiles, sit at 72.1 and 81.8 cm.

Since 12 months, the median has risen by 1.2 cm in one month. By 14 months it reaches 78.0 cm. That monthly change is less than half the 4.8 cm of the first month, because growth slows steadily through the first two years.

For girls at 13 months the median is 75.2 cm, 1.7 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 13 months is 76.7 cm. A boy exactly on the WHO median would read as about the 53rd percentile there, one reason two charts can give different answers for the same length.

Reading the table: a boy at 13 months who measures 78.0 cm is at about the 67th percentile, meaning that out of 100 boys this age, about 67 measure less. Under 2, this is length measured lying down. If a child under 2 was measured standing, WHO adds 0.7 cm before comparing.