Measurements are in inches. 

Size/ Cm Approximate age Measurements
36-38 0-3m 14.2-15
40-42 3-6m 15.7-16.5
44-46 6-12m 17.3-18.1
48-50 1-3Y 18.9-19.7
52-54 4Y+ 20.5-21.3