I bought this from another site and the reviews all said that it ran REALLY small - like, order at least 2 sizes up.  My daughter is 11 weeks old and 50th percentile in head circumference.  I was debating between the 12 month and the 18 months.  (Yes, she's 11 WEEKS - as in, under 3 months ... and I consdiered 12 months and 18 months based on reviews elsewhere)

I ordered the 12 month size since no one on Amazon seemed to mention the size issue.

I'm definitely glad I ordered up.  The 12 month goes on without issue, but it's a nice snug fit.  I can't imagine smaller fitting.  And she really is just average head size.

Aside from the sizing issue - which isn't a problem for me because I ordered right - I LOVE this hat.  It is so soft and so stinkin' cute.  The "fur" inside is especially silky soft.  Much softer than it looks.  Love, love this.  Which I could order in two different colors, but that seems excessive because it's such a substantial hat and I don't think you'd need more than 1.