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Microcephaly is a condition in which a person's head size is much smaller than that of others of the same age and sex. Head size is measured as the distance around the top of the head. A smaller than normal size is determined using standardized charts.


Microcephaly most often occurs because the brain does not grow at a normal rate. The growth of the skull is determined by brain growth. Brain growth takes place while a baby is in the womb and during infancy.

Conditions that affect brain growth can cause smaller than normal head size. These include infections, genetic disorders, and severe malnutrition.

Genetic conditions that cause microcephaly include:

Other problems that may lead to microcephaly include:

Becoming infected with the Zika virus while pregnant can also cause microcephaly. The Zika virus is present in Brazil and other parts of South America, along with Mexico, Central America, and the Caribbean.

When to Contact a Medical Professional

Most often, microcephaly is diagnosed at birth or during routine well-baby exams. Talk to your health care provider if you think your infant's head size is too small or not growing normally.

Call your health care provider if you or your partner has been to an area where Zika is present and you are pregnant or thinking about becoming pregnant.

What to Expect at Your Office Visit

Most of the time, microcephaly is discovered during a routine exam. Head measurements are part of all well-baby exams for the first 18 months. Tests take only a few seconds while the measuring tape is placed around the infant's head.

The provider will keep a record over time to determine:

  • What is the head circumference?
  • Is the head growing at a slower rate than the body?
  • What other symptoms are there?

It may also be helpful to keep your own records of your baby's growth. Talk to your provider if you notice that the baby's head growth seems to be slowing down.

If your provider diagnoses your child with microcephaly, you should note it in your child's personal medical records.


Skull of a newborn
Ultrasound, normal fetus - ventricles of brain


Centers for Disease Control and Prevention. Zika virus. Accessed February 24, 2016.

Johansson MA, Mier-Y-Teran-Romero L, Reefhuis J, Gilboa SM, Hills SL. Zika and the risk of microcephaly. N Engl J Med. 2016 May 25. PMID: 27222919.

Kinsman SL, Johnston MV. Congenital anomalies of the central nervous system. In: Kliegman RM, Stanton BF, St Geme JW, Schor NF, eds. Nelson Textbook of Pediatrics. 20th ed. Philadelphia, PA: Elsevier; 2016:chap 591.

Mirzaa G, Ashwal S, Dobyns WB. Disorders of brain size. In: Swaiman K, Ashwal S, Ferriero DM, Ferriero D, eds. Swaiman's Pediatric Neurology: Principles and Practice. 5th ed. Philadelphia, PA: Elsevier Saunders; 2012:chap 25.

Review Date: 
Reviewed By: 
Denis Hadjiliadis, MD, MHS, Associate Professor of Medicine, Pulmonary, Allergy, and Critical Care, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team. Editroial update 6/18/2016.