it is more correct to compare with the height:
(Unit：CM)
59=0-3M：full length=54 bust=66, fits for: 55-59cm height / 0-3 Months

66=3-6M：full length=58 bust=68, fits for: 59-66cm height / 3-6 Months

73=6-9M：full length=62 bust=72, fits for: 66-73cm height / 6-9 Months

80=9-12M：full length=65 bust=76, fits for: 70-80cm height / 9-12 Months
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.