When the infant’s size measurement falls below the 3rd centile. Children are underweight if their weight is below the 3rd centile. Similarly, children are shorter than normal if their height (length) falls below the 3rd centile. They have smaller heads than normal if their head circumference falls below the 3rd centile.
3% of normal children fall above the 97th centile and 3% below the 3rd centile. Therefore, the size of 6% of children falls outside the normal (i.e. average) range, i.e. above the 97th or below the 3rd centiles. Note that ‘normal’ refers to the average range of size for age. Therefore, some perfectly healthy children might fall outside the 3rd and 97th centiles.
Recently Z-scores (standard deviations from the mean) have also been used to assess a child’s size. A Z-score of −2 is equivalent to the 3rd centile.