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Microcephaly

Definition

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 of head circumference.

Causes

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:

• Cornelia de Lange syndrome

• Cri du chat syndrome

• Down syndrome

• Rubinstein-Taybi syndrome

• Seckel syndrome

• Smith-Lemli-Opitz syndrome

• Trisomy 18

• Trisomy 21

Other problems that may lead to microcephaly include:

• Uncontrolled phenylketonuria (PKU) in the mother

• Methylmercury poisoning

• Congenital rubella

• Congenital toxoplasmosis

• Congenital cytomegalovirus (CMV)

• Use of certain chemicals during pregnancy, especially alcohol and phenytoin

Becoming infected with the Zika virus while pregnant can also cause microcephaly. The Zika virus has been found in Africa, the South Pacific, tropical regions of Asia, and 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 child's health care provider if you think your infant's head size is too small or not growing normally.

Contact your child's 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. This takes only a few seconds while a 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 child's provider if you notice that the baby's head growth seems to be slowing down.

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

References

Antoniou E, Orovou E, Sarella A, et al. Zika virus and the risk of developing microcephaly in infants: a systematic review. Int J Environ Res Public Health. 2020;17(11):3806. PMID: 32471131 pubmed.ncbi.nlm.nih.gov/32471131/.

Centers for Disease Control and Prevention website. About Zika. www.cdc.gov/zika/about/index.html. Updated January 30, 2025. Accessed October 8, 2025.

Dobyns WB. Microcephaly and megalencephaly: disorders of brain size. In: Ashwal S, Pearl PL, eds. Swaiman's Pediatric Neurology: Principles and Practice. 7th ed. Philadelphia, PA: Elsevier; 2026:chap 43.

Trowbridge SK, Yang E, Yuskaitis CJ. Congenital anomalies of the central nervous system. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 631.

Review Date: 10/1/2025

Reviewed By: Charles I. Schwartz, MD, FAAP, Clinical Assistant Professor of Pediatrics, Regional Medical Director of Penn Medicine Primary and Specialty Care, Perelman School of Medicine at the University of Pennsylvania, General Pediatrician at PennCare for Kids, Phoenixville, PA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.