Proper fitting footwear is essential to healthy foot development. Determining the correct fit and size corresponds directly to your baby's developmental stage and activity level. Use this convenient printable foot size guide and beneficial information as a reference tool to help insure the best fit when purchasing footwear. Babies feet grow quickly. As a general rule, measure feet every month for babies under one year, every two months for one to two-year olds and every three months for two to three-year olds.