I have purchased these pjs for my baby since she was tiny. She's about to be 2 yo old now and is a bit of a pot belly :) so she is in the size 4 toddler. In most baby clothing, she is a size 3 toddler. The size 4 in these pjs fit her well since these are made to be snug-fitting (cotton, snug fit reduces flamability concerns according to the label in the packaging). The weight of the fabric is substantial, especially for the price. These thicker cotton pjs work especially well if you use the toddler Sleep Sack which comes in a flimsy fabric that doesn't do much to retain warmth. These pjs and socks make a perfect combo with the thin Sleep Sack.