it is more correct to compare with the height:
(Unit：CM)
70=3M-6M：length=32 bust=71 shoulder sleeve=25, fits for: 59-66cm height / 3-6 Months

80=9M-12M：length=35 bust=74 shoulder sleeve=27, fits for: 66-73cm height / 9-12 Months

90=18M：length=38 bust=77 shoulder sleeve=29, fits for: 73-80cm height / 12-18 Months

100=24M：length=41 bust=80 shoulder sleeve=31, fits for: 80-90cm height / 18-24 Months

110=3T：length=44 bust=83 shoulder sleeve=33, fits for: 90-100cm height / 3 Years Old

120=4T：length=47 bust=86 shoulder sleeve=35, fits for: 100-110cm height / 4 Years Old
Note:
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, 1 inch=2.540cm
3.Please check the figures carefully and order the right size, don't completely depend on the " age advice". It's just for reference.