Babies have thinner skin that is more vulnerable to sunburn than adult skin and special care must be taken to ensure that it's well protected from the sun, our solid flap sun protection hat is made out of a water-resistant upf 50+ fabric, we have included a tie closure under the chin except on the just born size.How to measure head circumference:Measure around the most prominent part of your baby's forehead,just above the eyebrows,around the widest part the back of head.