it is more correct to compare with the height:
Unit：CM
70=6M：length=40 bust=56 sleeve=32, fits for : 63-70cm height / 6 Months

80=12M：length=43 bust=58 sleeve=34, fits for : 70-80cm height / 12 Months

90=24M：length=46 bust=62 sleeve=36, fits for : 80-90cm height / 24 Months

100=3T：length=49 bust=64 sleeve=38, fits for : 90-100cm height / 3 Years old

110=4T：length=52 bust=66 sleeve=40, 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.