Ultrasonographic Definition of Fetal Microcephaly
Fetal microcephaly is ultrasonographically defined as a fetal head circumference (HC) measuring ≥3 standard deviations (SD) below the mean for gestational age, though this conventional definition results in substantial over-diagnosis of actual microcephaly at birth. 1, 2, 3
Standard Diagnostic Criteria
The most widely accepted prenatal ultrasound criterion for fetal microcephaly is a HC ≥3 SD below the mean for gestational age, originally established by Chervenak et al. in 1984 and still endorsed by the Society for Maternal-Fetal Medicine (SMFM). 3
This 3 SD cutoff remains statistically valid when applied to modern ultrasound equipment and current populations, with no significant difference in gestational age-specific mean HC measurements compared to historical data. 3
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. 4
Critical Limitations of Prenatal Diagnosis
The conventional prenatal definition has a high false-positive rate, with only 57% of fetuses diagnosed with microcephaly in utero actually having confirmed microcephaly at birth. 2
Using the Jeanty reference range (the conventional standard), the positive predictive value (PPV) for microcephaly at birth is only 57.1%, meaning 43% of cases are over-diagnosed. 1, 2
Alternative reference ranges (INTERGROWTH-21st and Israeli references) do not significantly improve diagnostic accuracy, with PPVs of 61.5% and 66.7% respectively. 2
The prenatal HC deviation from the mean is significantly over-estimated compared to the actual postnatal OFC deviation, with differences ranging from -0.74 SD to -1.95 SD depending on the reference used. 1, 2
Improving Diagnostic Accuracy
To reduce false-positive diagnoses, integrate additional clinical parameters beyond HC measurement alone:
Associated malformations: The presence of other structural abnormalities significantly increases the likelihood of true microcephaly at birth. 1, 2
Small-for-gestational age (SGA): Concurrent growth restriction supports the diagnosis. 1
Proportionality ratios: Decreased HC/abdominal circumference (AC) and HC/femur length (FL) ratios indicate disproportionate head growth. 1
Family history: A positive family history of microcephaly increases diagnostic certainty. 1, 2
Foramen magnum-to-cranium distance (FCD): This vertical cranial measurement can exclude false-positive cases due to acrocephalic-like head deformation, improving PPV to 78% when combined with HC criteria. 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. 5, 4
Mild microcephaly is defined as OFC between -2 and -3 SD, while severe microcephaly is <-3 SD below the mean. 4, 6
Microcephaly represents a clinical finding rather than a specific diagnosis, with outcomes depending on the underlying etiology and associated brain abnormalities on imaging. 5
Important Caveats
Craniosynostosis exception: Small HC may occur with normal brain size due to premature suture fusion, representing microcephaly without true micrencephaly. 4, 7
Serial measurements are more valuable than single measurements for assessing growth trajectories and confirming the diagnosis. 8
The degree of prenatal microcephaly severity is consistently over-estimated compared to postnatal measurements, which should inform counseling. 1, 2