Microcephaly is a condition characterized by a head circumference that is smaller than normal, specifically three standard deviations below the mean head size for a gestational age. Microcephaly is also diagnosed when there is an abnormal ratio between the head circumference and the length of the femur or abdomen. Microcephaly can have a variation of causes, ranging from genetic to infectious to environmental. The incidence rate of microcephaly is estimated to range from 2 in 10,000 to 12 in 10,000 live births. This incidence rate does not take into account the cases of pregnancy loss and stillbirth.
When microcephaly is suspected, a detailed ultrasound of the fetal anatomy is recommended to scan for additional anomalies. Fetal karyotype testing is recommended to rule out aneuploidy, or chromosomal abnormalities. A detailed family history is important, especially in cases of isolated microcephaly, to provide insight to underlying causes. 20-33% of microcephaly cases are considered genetic rather than environmental.
The prognosis and recurrence risk for microcephaly depend on the underlying cause of the condition and the associated malformations. When microcephaly is associated with other abnormalities, the prognosis is poor. When chromosomal abnormalities are the underlying cause of microcephaly, the recurrence rate is estimated to be 1% or the maternal age-related risk, whichever is greater.