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.

These conditions cause microcephaly:

• Cornelia de Lange syndrome
• Cri du chat syndrome
• Down syndrome
• Rubinstein-Taybi syndrome
• Seckel syndrome
• Smith-Lemli-Opitz syndrome
• Trisomy 13
• Trisomy 18

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
• Malnutrition