To knit the Preemie size, use Sport weight yarn (per pattern), or DK.
Pattern is on pg 11 of the Care Wear Pattern and Information Booklet, 2018 Edition, HERE.

Jane said that she has made hundreds of these hats in various sizes for the NICU at St. Joseph’s Hospital in Tampa and the nurse manager there is the one who named them “The Perfect Preemie Hat”.

Because these preemie hats stretch but keep their shape so well, the ribbing can be folded up to fit the tiniest babies, and the cuff adjusted as the infant’s head grows.

This hat pattern can be graded to any size -- from children to adults -- using larger needles and worsted weight yarn, increasing the number of sts and length as needed.