I bought this for my 9-month-old daughter to wear for a photo shoot. I wasn't sure which size she would wear, so I ordered the 9-12 mo and 12-18 mo. She is average size, and both were pretty small on her. I actually didn't see much difference between the two. There was no way she could ever wear them. The "romper" was more of a shirt/top, and she would need to wear pants underneath because it was so short. It was really cute to take photos in, but was not functional other than that. For spending $30+, I was pretty disappointed and returned it.