it is more correct to compare with the height:
(Unit：CM)
3M=：length=41 bust=52 waist=49, fits for 0-3 Months

6M=：length=44 bust=53 waist=50, fits for 3-6 Months

12M=：length=47 bust=54 waist=51, fits for 6-12 Months

18M=：length=51 bust=56 waist=53, fits for 12-18 Months

24M=：length=55 bust=58 waist=55, fits for 18-24 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.