Definition of Fetal Macrocephaly on Prenatal Ultrasound
Fetal macrocephaly is defined as a head circumference (HC) exceeding 2 standard deviations (SD) above the mean for gestational age, which corresponds to above the 97th-98th percentile. 1, 2
Standard Diagnostic Threshold
The primary diagnostic criterion is HC >2 SD above the mean (>97th percentile) for gestational age and sex, which represents the standard definition used in prenatal ultrasound assessment. 1, 2
Mild macrocephaly is defined as HC between 2 and 3 SD (between 97th and 99.7th percentile) above the mean. 1
Clinical Significance of Degree
Mild macrocephaly (HC ≤2.5 SD) carries a good prognosis, especially when one parent has macrocephaly with normal development, and this degree of enlargement is often familial and benign. 3, 2
Syndromic macrocephaly typically presents with HC >2.5 SD above the mean, and fetuses with associated anomalies have significantly larger head circumferences than those with isolated macrocephaly. 3
Important Diagnostic Considerations
Macrocephaly vs. Megalencephaly
Macrocephaly refers to a large head size, while megalencephaly specifically indicates increased brain tissue volume. 2, 4
Macrocephaly can be secondary to multiple causes including hydrocephalus, enlarged extracerebral spaces, intracranial cysts, cerebral edema, thickened calvarium, or brain tumors—not just brain overgrowth. 2, 4
Timing and Associated Findings
Macrocephaly associated with other anomalies is typically diagnosed earlier (around 28 weeks) compared to isolated macrocephaly (around 32 weeks). 3
When fetal macrocephaly is associated with brain or systemic anomalies, syndromic macrocephaly can be diagnosed prenatally, and associated CNS findings may include ventriculomegaly, malformations of cortical development, callosal abnormalities, overdeveloped sulcation, or large subarachnoid spaces. 3
Critical Caveats
Exercise caution due to limitations in HC measurement accuracy, including lack of population-specific nomograms and inconsistencies between prenatal and postnatal growth curves. 2
Serial measurements are more valuable than single measurements for assessing growth trajectories and confirming the diagnosis. 5
Parental head circumference should be obtained, as positive family history of macrocephaly with normal development is reassuring, while normal parental HC warrants more thorough investigation. 2