Head circumference for boys at 1 month
At 1 month, the median head circumference for boys on the WHO Child Growth Standards is 37.3 cm (14.7 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 36.5 and 38.1 cm, a spread of 1.6 cm. The outer lines of the chart, the 2nd and 98th percentiles, sit at 34.9 and 39.6 cm.
Since birth, the median has risen by 2.8 cm in one month. By 2 months it reaches 39.1 cm.
For girls at 1 month the median is 36.5 cm, 0.7 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 1 month is 38.2 cm. A boy exactly on the WHO median would read as about the 30th percentile there, one reason two charts can give different answers for the same head circumference.
Reading the table: a boy at 1 month who measures 37.5 cm is at about the 58th percentile, meaning that out of 100 boys this age, about 58 measure less.
Percentiles at 1 month
Download CSV (all ages)| Percentile | z-score | Metric | Imperial |
|---|---|---|---|
| 2nd | −2.00 | 34.9 cm | 13.8 in |
| 5th | −1.64 | 35.4 cm | 13.9 in |
| 10th | −1.28 | 35.8 cm | 14.1 in |
| 25th | −0.67 | 36.5 cm | 14.4 in |
| 50th (median) | 0.00 | 37.3 cm | 14.7 in |
| 75th | +0.67 | 38.1 cm | 15.0 in |
| 90th | +1.28 | 38.8 cm | 15.3 in |
| 95th | +1.64 | 39.2 cm | 15.4 in |
| 98th | +2.00 | 39.6 cm | 15.6 in |
WHO charts label z = ±2 as the 2nd and 98th percentiles (exactly 2.3rd and 97.7th), the cutoffs CDC recommends on WHO charts. Values computed with the LMS method from the published tables (retrieved 2026-10-05).