I bought the 6-12 months the first time because reviewers said it was sized too small...I had to buy the 0-6 months also because 6-12 months fit on my toddler better than on my newborn. The 0-6 months one is still a little large for my newborn but I still use it, he is 4 weeks old now. Only way to be sure it to try it out for yourself.