Most Common Causes of Fetal Microcephaly
The most common causes of fetal microcephaly are genetic disorders (accounting for approximately 59-70% of cases), followed by congenital infections (particularly cytomegalovirus), intrauterine teratogen exposure (especially alcohol), and brain ischemia/hypoxia from placental insufficiency or birth complications. 1, 2
Breakdown by Etiology
Genetic Causes (59-70% of cases)
Genetic factors represent the dominant cause of fetal microcephaly and include:
- Chromosomal abnormalities: Aneuploidies and copy number variants (CNVs), including submicroscopic deletions/duplications 1
- Single gene disorders: A rapidly growing number of monogenic causes affecting neuronal proliferation, DNA repair pathways, and brain development 1, 3
- Specific gene mutations: WDR62 (most common gene associated with microcephaly and polymicrogyria), genes involved in microtubule function (TUBB, KATNB1, CENPJ), and neuronal proliferation genes 1, 3
Non-Genetic Causes (30-41% of cases)
Non-genetic causes account for almost 30% of microcephaly cases 1 and include:
Congenital Infections (8-12% of identified cases)
- Cytomegalovirus (CMV): The most frequent non-genetic cause, accounting for 12-31% of cases with polymicrogyria and microcephaly 1
- Look for: intracranial calcifications, white matter abnormalities, chorioretinitis, congenital sensorineural hearing loss
- In symptomatic CMV newborns, 19.5% present with microcephaly 1
- Zika virus: Associated with primary microcephaly and spectrum of brain malformations, with rates as high as 58 per 1000 live births in exposed pregnancies 1, 4
- Look for: macular, retinal, and optic nerve abnormalities
- Other TORCH infections: Toxoplasmosis, rubella, varicella zoster, herpes simplex virus 1
Teratogen Exposure
Perinatal/Postnatal Brain Injuries (10% of cases)
- Placental insufficiency
- Birth complications and hypoxic-ischemic injury
- Postnatal infarcts and concussions 1, 2
Vascular Disruptions
- Monochorionic twin complications (twin-to-twin transfusion syndrome, death of co-twin)
- Prenatal trauma or near miscarriage 1, 5
Critical Diagnostic Clues
History Red Flags
- Family recurrence: Suggests genetic cause 1
- Consanguinity: Increases risk of autosomal recessive disorders 1
- Maternal fever/rash during pregnancy: Suggests congenital infection 1
- Twinning history: Risk factor for vascular disruption 1
- Preterm birth: 7.7-fold increased relative risk 2
- British Asian ethnicity: 3.6-fold increased relative risk 2
Imaging Findings to Scrutinize
- Calcifications: Indicative of fetal injury (CMV, toxoplasmosis) or pseudo-TORCH syndrome (COL4A1/COL4A2 mutations) 1
- Gliosis, cysts: Signs of fetal injury 1
- White matter hyperintensities: Suggestive of CMV 1
- Polymicrogyria: Can be caused by both genetic factors and fetal injury 1
Associated Findings
- Ophthalmological abnormalities: Found in up to 48% of microcephaly cases 1
- Perform detailed eye examination in every case
- Dysmorphic features and congenital abnormalities outside CNS: Indicative of genetic syndrome 1
Important Caveats
Microcephaly at birth is not always present - postnatal microcephaly can develop during the first year of life even with normal head size at birth, particularly after congenital Zika infection 6. Serial head circumference measurements are essential, as deceleration of growth rate may be the only early clue 7.
Up to 19% of cases remain undetermined despite extensive workup 2, and the diagnostic yield heavily depends on expert MRI evaluation combined with targeted genetic testing 1.
De novo mutations account for 50.6% of pathogenic genetic alterations, and up to 75.1% cannot be detected by routine prenatal screening 3, emphasizing the importance of advanced genetic testing (chromosomal microarray analysis and whole-exome sequencing) when standard workup is unrevealing.