Believe the cookie. They will wear a lot of T Shirts.
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AS Colour 100% Cotton Baby Onesie. Made from super soft 190gsm cotton with a superfine cotton rib. Constructed with tonal shoulder domes, shoulder to shoulder tape for durability and preshrunk to minimise any shrinkage

Size	Body Width (cm)	Body Length (cm)
0 - 3M	19	36
3 - 6M	20.5	39
6 - 12M	22	42
12 - 18M	23.5	45
18 - 24M	25	48

 

How to Measure:

• Lay an existing t-shirt flat.
• Body Width: Measure 3cm below armpit, from one side to the other.
• Body Length: Measure from the top of the shoulder to the bottom of the t-shirt.