Minor differences in length between left and right legs is not uncommon. Difference of a centimeter or so is probably of little significance. Discrepancy of more than 2cm can lead to tilting of the pelvis and spine and may cause backache in the long term.
It is important to verify that any perceived difference is real since primary spinal deformity (scoliosis) can produce an apparent leg length difference when non exists. Leg length can be accurately measured with special X rays or CT scans.
Leg length discrepancy can result from a number of different conditions. The most important one to rule out is a dislocated hip since this will require prompt surgical treatment. Other causes include damage to growth plates and neuromuscular conditions such as cerebral palsy. Sometimes the deformity occurs before birth. Fibular and tibial hemimelia are examples of such conditions. These are rare and vary in severity such that milder cases may not be obvious until the child is several years old.
Several charts are available which can predict the expected limb length discrepancy at skeletal maturity based on the patients age at and discrepancy at presentation. These help decision making as to the most appropriate form of management. The options are orthotics (shoe raise) or surgery either to lengthen the short leg or shorten the long leg. If the predicted difference at skeletal maturity is 2cm or less non-operative management is appropriate. A small raise can be inserted into the shoe without cosmetic issues. Larger differences often merit surgical correction.