Our daughter has run on thin side since she was born (always high % in length and low % in weight, she's a string-bean like her daddy), but this thing is tight (and I mean we've had it a few weeks and I'm not sure how much longer she can wear it) and all her other clothes in the same range (3-6 months) are big on her still. So going based on the size of her other clothes I would say these run small compared to every brand and are almost exactly one size off Carter's Just One You. Otherwise, nice construction, soft material and cute as a button!