Microcephaly is a head size (measured as the distance around the top of the head) significantly below the median for the infant's age and sex. Significantly below is generally considered to be smaller than three standard deviations below the mean, or less than 42 cm in circumference at full growth. It most often occurs because of failure of the brain to grow at a normal rate.
Review Date 12/10/2021
Updated by: Neil K. Kaneshiro, MD, MHA, Clinical Professor of Pediatrics, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.