Merck Manual

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Stephen J. Falchek

, MD, Nemours/Alfred I. duPont Hospital for Children

Reviewed/Revised Jul 2023
Topic Resources

Microcephaly is an abnormally small head. Often the head is small because the brain is small and abnormally developed.

  • Microcephaly can be caused by many disorders, including genetic abnormalities, infections, and brain defects, or can sometimes run in families.

  • Newborns with severe microcephaly usually have symptoms of brain damage.

  • Diagnosis is made before birth by doing ultrasound tests or after birth by measuring the head circumference.

  • Doctors usually do imaging tests to look for brain abnormalities and sometimes blood tests to look for a cause.

  • Because microcephaly can range from mild to severe, treatment options can range as well.

  • Early intervention may be especially helpful.

The size of the head is typically determined by the size of the brain. Thus, decreased growth of the brain, or part of the brain, is the cause of microcephaly. Microcephaly can occur alone or in combination with other major birth defects and can be present at birth or develop later in infancy.

Microcephaly is not a common condition.

Causes of Microcephaly

Microcephaly can be caused by a number of disorders, including

Familial microcephaly, in which a smaller head size runs in the child's family, is normally mild and does not cause neurologic symptoms.

Symptoms of Microcephaly

Diagnosis of Microcephaly

  • Before birth, ultrasound

  • After birth, physical examination and imaging tests

  • Genetic testing

After birth, doctors measure a baby's head circumference Head Circumference Physical growth refers to an increase in body size (length or height and weight) and in the size of organs. From birth to about age 1 or 2 years, children grow rapidly. After this rapid infant... read more (the measurement of the head around its largest area) during routine physical examinations. They diagnose microcephaly when the head circumference is significantly smaller than the normal range for babies of the same sex, age, and ethnic group in the region where the baby lives. Occasionally, the diagnosis is made when the baby's head circumference began in the normal range but is not appropriately increasing as the baby ages.

When making the diagnosis, doctors also take into account the head circumference of the baby's parents because a slightly small head size may run in the family (a condition called benign familial microcephaly).

A baby who has this birth defect may be evaluated by a geneticist. A geneticist is a doctor who specializes in genetics (the science of genes and how certain qualities or traits are passed from parents to offspring). Genetic testing of a sample of the baby's blood may be done to look for chromosome and gene abnormalities. This testing can help doctors determine whether a specific genetic disorder is the cause and rule out other causes.

Treatment of Microcephaly

  • Treatment of symptoms

  • Intervention for physical and intellectual problems

Microcephaly is a lifelong condition, and there is no cure or standard treatment.

Symptoms resulting from brain damage are treated. Some disorders causing microcephaly can be treated.

Regular check-ups and follow-ups by a care team are very important. Developmental services, known as early intervention, often help babies with microcephaly maximize their physical and intellectual abilities.

If a smaller head size runs in the child' family, treatment generally is not necessary.

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

  • March of Dimes: An organization for pregnant people and babies that provides support and information about how to prevent maternal health risks, premature birth, and mother and infant deaths

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