Length for girls at 18 months

At 18 months, the median length for girls on the WHO Child Growth Standards is 80.7 cm (31.8 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 78.7 and 82.7 cm, a spread of 3.9 cm. The outer lines of the chart, the 2nd and 98th percentiles, sit at 74.9 and 86.5 cm.

Since 17 months, the median has risen by 1.0 cm in one month. By 19 months it reaches 81.7 cm. That monthly change is less than half the 4.5 cm of the first month, because growth slows steadily through the first two years.

For boys at 18 months the median is 82.3 cm, 1.6 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 18 months is 80.3 cm. A girl exactly on the WHO median would read as about the 55th percentile there, one reason two charts can give different answers for the same length.

Reading the table: a girl at 18 months who measures 82.0 cm is at about the 67th percentile, meaning that out of 100 girls 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.