Do not refer only to the age of the child to choose the size of his clothes, you should take into account its actual measurements because all babies do not grow the same way and at the same pace. Below are indicative measures that will help you to choose the right size:
Clothes
MAMY FACTORY'S SIZESABC
  cm inches cm inches cm inches
1M 54 21 37 14,5 37 14,5
3M 60 23,5 40 16 40 16
6M 67 26,5 44 17,5 44 17,5
12M 74 29 46 18 46 18
18M 81 32 47 18,5 48 19
24M 86 34 48 19 52 20,5
36M 94 37 50 20 54 21
ABCD
Child height Waist Chest Head perimeter
Caps / hats
MAMY FACTORY'S SIZESD
  cm inches
T1 (0-1M) 39-41 15,5-16
T2 (3-6M) 43-45 17-18
T3 (12-24M) 47-49 18,5-19,5
T4 (24-36M) 51-53 20-21

GlovesSlippers / Shoes
MAMY FACTORY'S SIZESAGEMAMY FACTORY'S SIZESSIZES
      EU UK
T1 1-6M T1 17 / 20 1-3
T2 12-24M T2 22 / 24 4-6