Microcephaly is defined by a head circumference, as measured around the glabella and occipital protuberance [OFC], that is below the 3rd percentile for age (gestational age if congenital), sex and race. Proportions among head dimensions (length and breadth) are retained, microcephaly being caused by a small brain. Where the head shape is abnormal, as in certain forms of craniosynostosis, the head circumference is not a valid index of cranial volume and does not correlate with microcephaly.