Microcephaly is a medical finding characterised by a cephalic demarcation that is smaller than that announced for age and gender. They can occur at birth (congenital microcephaly) or after birth (secondary microcephaly). When in microcephaly the growth of the brain does not follow the growth of the rest of the newborn's body (somatic development) it is then known as disproportionate microcephaly and when the head circumference is less than -3 standard deviations for the gestational date and gender it is signalled as severe microcephaly. There are many causes of microcephaly, but intrauterine infections are the most common, such as syphilis, rubella, cytomegalovirus, herpes simplex virus and, more recently, the Zika virus. At the end of 2015, there was an unforeseen increase in the number of microcephaly cases in newborns in Brazil, the main conjecture being the abrupt spread of the Zika virus.