From birth to age 5 years, the sitting height increases by 27 cm. The sitting height represents 66% of the standing height at birth and 56% at age 5 years. The T1 to S1 segment must still grow 15 cm at age 5 years. The deficit in sitting height caused by a perivertebral arthrodesis can be calculated with accuracy; a practical diagram can be used. At age 5 years, the spinal canal has grown to ∼95% of its definitive size. Therefore, perivertebral arthrodesis will have no influence on the size of the spinal canal. All indicators do not progress at the same speed. The increase in weight and the thoracic volume remain offset relative to the other indicators.