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 3 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.
- Reviewed last on: 11/9/2007
- Deirdre O’Reilly, MD, MPH, Neonatologist, Division of Newborn Medicine, Children’s Hospital Boston and Instructor in Pediatrics, Harvard Medical School, Boston, Massachusetts.
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