Length for boys at 4 months

At 4 months, the median length for boys on the WHO Child Growth Standards is 63.9 cm (25.2 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 62.5 and 65.3 cm, a spread of 2.8 cm. The outer lines of the chart, the 2nd and 98th percentiles, sit at 59.7 and 68.0 cm.

Since 3 months, the median has risen by 2.5 cm in one month. By 5 months it reaches 65.9 cm. That monthly change is smaller than the 4.8 cm of the first month, because growth slows steadily through the first two years.

For girls at 4 months the median is 62.1 cm, 1.8 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 4 months is 63.1 cm. A boy exactly on the WHO median would read as about the 61st percentile there, one reason two charts can give different answers for the same length.

Reading the table: a boy at 4 months who measures 64.5 cm is at about the 62nd percentile, meaning that out of 100 boys this age, about 62 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.