I love Gerber Sleep and Plays for my babies in the summer. The cloth is thin, but not cheap. I just got these for my third little one, now 5 months old. They do run small. My guess would be somewhere between a whole to half size smaller. My son's 3-6 month clothes are starting to look a little small, and these fit perfectly (6-9 month size), but other 6-9 month body suits I have for him are still too big. I highly reccommend these for the summer, but keep in mind that they run small. Since I bought my 8 year old son them as a baby they ran small.