I purchased this for my baby, who is now almost 5 months old. I ordered it in the 3-6 size. My daughter is very small, she only weighs about 12.5 pounds (10-25th percentile), 24 in. tall (just less than 50th percentile), and her head measures in the 5th percentile. These were measured by our doctor. I put the outfit on her yesterday, and the opening for her head was SO tight! I tried stretching it before putting it on her, as I do with all of her other shirts, and that didn't seem to do much. As it went on her, she started screaming because it got stuck on her head, it was so tight. I can't believe that the designer would make it like this. I don't know how everyone else got it on their daughters without a problem, especially when their heads are even bigger than my daughters. Her head is pretty small and it was still so tight on her. Once it was on her, it looked incredibly cute. The rest of the outfit fit her pretty well. However, because of the tight opening on the shirt, I wouldn't recommend it to others. I probably won't be putting it on her again, either. What a waste of my money. Be warned!