Fact sheet
Updated 14 October 2016

Key facts

  • Microcephaly is a condition where a baby is born with a small head or the head stops growing after birth.
  • Microcephaly is a rare condition. One baby in several thousand is born with microcephaly.
  • The most reliable way to assess whether a baby has microcephaly is to measure head circumference 24 hours after birth, compare the value with WHO growth standards, and continue to measure the rate of head growth in early infancy.
  • Babies born with microcephaly may develop convulsions and suffer physical and learning disabilities as they grow older.
  • There are no specific tests to determine if a baby will be born with microcephaly, but ultrasound scans in the third trimester of pregnancy can sometimes identify the problem.
  • There is no specific treatment for microcephaly.


Microcephaly is a condition where a baby has a head size much smaller compared with other babies of the same age and sex. Head size is an important measurement to monitor a child’s brain growth. The severity of microcephaly ranges from mild to severe. Microcephaly can be present at birth (congenital) or may develop postnatally (acquired).

Scope of the problem

Microcephaly is a rare condition. Reported estimate incidence of microcephaly has wide variation due to the differences in the definition and target population.

Increased number or clustering of cases of microcephaly have been reported in context of outbreaks of Zika virus infection. The most likely explanation of available evidence is that Zika virus infection during pregnancy is a cause of congenital brain abnormalities including microcephaly.

In addition to microcephaly, a range of manifestations of varying severity has been reported among newborns that were exposed to Zika virus in utero. These include malformations of the head, seizures, swallowing problems, hearing and sight abnormalities. Other outcomes associated with Zika virus infection in utero may involve miscarriages and stillbirths. Together, this spectrum is referred to as ‘congenital Zika virus syndrome.’


Early diagnosis of microcephaly can sometimes be made by fetal ultrasound. Ultrasounds have the best diagnosis possibility if they are made at the end of the second trimester, around 28 weeks, or in the third trimester of pregnancy. Often diagnosis is made at birth or at a later stage.

Babies should have their head circumference measured in the first 24 hours after birth and compared with WHO growth standards. The result will be interpreted in relation to the gestational age of the baby, and also the baby’s weight and length. Suspected cases should be reviewed by a paediatrician, have brain imaging scans, and have their head circumference measured at monthly intervals in early infancy and compared with growth standards. Doctors should also test for known causes of microcephaly.

Causes of microcephaly

There are many potential causes of microcephaly, but often the cause remains unknown. The most common causes include:

  • infections in the womb: toxoplasmosis (caused by a parasite found in undercooked meat), rubella, herpes, syphilis, cytomegalovirus, HIV and Zika virus;
  • exposure to toxic chemicals: maternal exposure to heavy metals like arsenic and mercury, alcohol, radiation, and smoking;
  • pre- and perinatal injuries to the developing brain (hypoxia-ischemia, trauma);
  • genetic abnormalities such as Down syndrome; and
  • severe malnutrition during fetal life.

Signs and symptoms

Many babies born with microcephaly may demonstrate no other symptoms at birth but go on to develop epilepsy, cerebral palsy, learning disabilities, hearing loss and vision problems. In some cases, children with microcephaly develop entirely normally.

Treatment and care

There is no specific treatment for microcephaly. A multidisciplinary team is important to assess and care for babies and children with microcephaly. Early intervention with stimulation and play programmes may show positive impacts on development. Family counselling and support for parents is also extremely important.

WHO response

WHO has been working closely with countries affected by Zika virus and related complications on the investigation of and response to the outbreak since mid-2015.

The Strategic Response Framework and Joint Operations Plan outlines steps that WHO is taking with partners to respond to Zika and potential complications.

  • Working closely with affected countries on the Zika outbreak investigation and response and on the unusual increase in microcephaly cases.
  • Engaging communities to communicate the risks associated with Zika virus disease and how they can protect themselves.
  • Providing guidance and mitigating the potential impact on women of childbearing age and those who are pregnant, as well as families affected by Zika virus.
  • Helping affected countries strengthen care for pregnant women and the families of children born with microcephaly.
  • Investigating the reported increase in microcephaly cases and the possible association with Zika virus infection by bringing together experts and partners.
  • Describing the full spectrum of congenital Zika virus syndrome, which may evolve, as part of the WHO Zika virus research agenda.


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