Microcephaly is a condition in which a person's head is significantly smaller than normal for their age and sex, based on standardized charts. Head size is measured as the distance around the top of the head.

Microcephaly most often occurs because the brain fails to grow at a normal rate. Skull growth is determined by brain growth. Brain growth takes place while in the womb and during infancy.

Conditions that affect brain growth can cause microcephaly. 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

These additional conditions may indirectly cause microcephaly:

• Uncontrolled phenylketonuria (PKU) in the mother
• Methylmercury poisoning
• Congenital rubella
• Congenital toxoplasmosis
• Congenital cytomegalovirus (CMV)
• Use of certain drugs during pregnancy, especially alcohol and phenytoin

Microcephaly is often diagnosed at birth or during routine well-baby examinations when the infant's height, weight, and head circumference is measured. If you suspect your infant's head size is too small or not growing normally, consult your health care provider.

Microcephaly is usually discovered by the health care provider during routine examination. Head measurements are part of all well-baby examinations up to 18 months (longer in certain circumstances). They are painless and take only a few seconds while the measuring tape is placed around the infant's head.

Documenting microcephaly in detail may include:

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

Note: A record of the head circumference should be maintained over time.

Although the health care provider maintains records on your baby, it may be helpful to maintain your own records, and bring them to the health care provider's attention if you notice that the infant's head growth pattern seems to be decreasing.

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

Kinsman SL, Johnston MV. Congenital anomalies of the central nervous system.In: Kliegman RM, Stanton BF, St. Geme JW III, Schor NF, Behrman RE, eds.Nelson Textbook of Pediatrics. 19th ed.Philadelphia,Pa: Saunders Elsevier; 2011:chap 585.10.