willaby sizing is based on average to slightly above average measurements for the age indicated on the size label.
 

Our clothing sizing corresponds to most other U.S. clothing brands as follows:

nb (newborn) = up to 3 months 
3m = 3-6 months
6m = 6-12 months
12m = 12-18 months

Approximate head circumferences for bonnet sizes:

nb = 13.5 to 14"
0-3m = 14 to 15.5"
3-6m = 15.5 to 17"
6-12m = 17 to 18.5"