Microcephaly describes the problems caused by a “small skull”. (The term “small” is used to refer to a head circumference, which is less than three standard deviations below the average for age, sex and race. Body size is also taken into account.)

A small skull can be caused if a baby in the womb is exposed to any X-rays at all in the first trimester or if there is lack of oxygen to the brain during or even some time after birth. Serious reduction of head size usually, but not always, results in mental retardation, as the reduced size of the skull prevents the proper development of brain formation and therefore causes brain damage. Microcephaly may be recognised at birth if the baby has a receding forehead, if the back of the head is flat or if the baby has notably small head measurements.