Here is the size chart for choosing Watery Fjord Baby Rashguard. For this rashguard, chest measurement and length are used to find your correct size.
If you happen to be between two sizes, choose the one that is closest or based on your preferred fit. You need to assess which fit you want; for a tight fit, choose the smaller of the two sizes, and conversely, choose the larger of the two sizes if you prefer a looser fit.