Fetal Microcephaly Diagnosis and Classification

Diagnostic Criteria

• The World Health Organization (WHO) defines postnatal microcephaly as an occipitofrontal circumference (OFC) ≥2 SD below the mean, which differs from the prenatal ultrasound threshold 1

Clinical Context and Classification

• Microcephaly is classified as primary (congenital) if present at birth or detected in utero, versus secondary (postnatal) if it develops after birth, according to the Nature Reviews Neurology 2, 1
• Mild microcephaly is defined as OFC between -2 and -3 SD, while severe microcephaly is <-3 SD below the mean, as stated by the Praxis Medical Insights 1
• Microcephaly represents a clinical finding rather than a specific diagnosis, with outcomes depending on the underlying etiology and associated brain abnormalities on imaging, as noted by the Nature Reviews Neurology 2

Important Caveats

• Craniosynostosis exception: Small HC may occur with normal brain size due to premature suture fusion, representing microcephaly without true micrencephaly, as reported by the Praxis Medical Insights 1, 3
• Serial measurements are more valuable than single measurements for assessing growth trajectories and confirming the diagnosis, according to the Praxis Medical Insights 4