Microcephaly refers to an abnormally small head defined as a head circumference (HC) of 3 standard deviations (SD) or more below the mean for the gestational age [1-4]. Microcephaly can be present at birth or it may develop in the first few years of life [6] . 

Prenatally, Chervenak et al. found fetal microcephaly could not be reliably diagnosed or excluded on the basis of a single ultrasound examination if the HC was between 2 SD and 5 SD below the mean  [2]. The Society for Maternal-Fetal Medicine (SMFM) recommends that a detailed neurosonographic examination be done on fetuses with an HC greater than 2 standard deviations below the mean to look for findings such as intracranial calcifications, cerebellar hypoplasia and other abnormalities. The finding of a sloping forehead should increase the index of suspicion for pathological microcephaly. The SMFM also  advises the diagnosis of pathologic microcephaly is considered certain when the fetal HC is equal to or greater than 5 SD below the mean for the gestational age [4]. A follow up ultrasound in 3 to 4 weeks is recommended for fetuses with an HC more than 2 SD below the mean.

Pathological microcephaly is usually associated with abnormal neurological findings , mental retardation, and sometimes seizures.  Causes of microcephaly include , but are not limited to, a chromosomal abnormality (trisomy or deletion),  a gene defect such as Smith-Lemli-Opitz, an infection such as rubella, cytomegalovirus (CMV),  toxoplasmosis, or possibly Zika virus, intrauterine fetal stroke, exposure to high doses of radiation, exposure to drugs such as alcohol, aminopterin ,or hydantoin, or a maternal condition such as phenylketonuria (PKU).