My daughter is on the small side of average and I bought her the 2/3 size when she was 3 and it fit her for about six months.  Now I'm back buying the 4/5 size.  I love these undies.  They are light weight, but well made.  They are pretty with the white and fancy edge - she loves them. They fit her well without riding up.  She is 32 pounds, 39 inches and the 2/3 size is just fitting - almost too small.  They will fit her for about another month or so I think.  They wash well, are breathable and wonderful! I did not buy the 4/5 size originally because I didn't want them falling off as we have a hard time finding pants that fit her. Love that they're organic cotton too.