As a new mom, it's hard to figure out sizing of onesies.  This was a 6 month outfit (which means it can fit 3-6 month olds usually).  Other 6 month onesies I have gotten seemed super tight on my 4 month old boy.  He's not very chubby though so it made me question how one can buy clothes for babies accurately.  I was happy with this purchase.  Not only does he look super cute in his Superman onesie, it appears that he'll be able to actually wear this one now and at 6 months of age.  Yay!