It is more correct to compare with the height:(Unit: CM)6/70=3M/6M: length=30 bust=58 sleeve=18, fits for: 50-55cm height/0-6 Months8/80=9M/12M: length=33 bust=62 sleeve=20, fits for: 55-65cm height/6-12 Months10/90=18 M: length=35 bust=64 sleeve=22, fits for: 65-75cm height/12-18 Months12/100=24 M: length=37 bust=66 sleeve=24, fits for: 75-85cm height/18-24 MonthsA notice:1. There is 2-3% difference according to manual measurement. (1 inch = 2.54 cm)2. All measurement in cm, please note 1cm=0.3937ch, 1inch=2.540cm3. Please check the numbers carefully and order the right size, not entirely dependent on the "age advice". It is for reference only.