Microcephaly is a condition where an infant’s head is significantly smaller than expected compared to the average size of children of the same age and sex. This reduced head size reflects a smaller underlying brain volume, often resulting from atypical brain development during gestation or early infancy. Detection relies heavily on standardized measurement methods applied both before and after birth to monitor growth trajectory. A systematic evaluation is necessary to establish a formal diagnosis.
Defining Microcephaly Through Measurement
Microcephaly is diagnosed by a statistical definition comparing an individual’s head size to population data, not by simple visual assessment. Healthcare providers use standardized growth charts, such as those published by the World Health Organization (WHO) or the Centers for Disease Control and Prevention (CDC), to plot the head circumference measurement. These charts establish the expected range of head sizes for a given gestational age, chronological age, and sex.
The diagnosis is typically assigned when the head circumference falls more than two standard deviations (SD) below the mean for the reference population. This threshold generally corresponds to a measurement at or below the third percentile. A measurement exceeding three standard deviations below the mean is often classified as severe microcephaly, which is associated with a higher likelihood of neurological complications.
Prenatal Detection Using Imaging
Initial detection of microcephaly before birth relies primarily on fetal ultrasound, typically performed as part of routine prenatal care. During this imaging procedure, trained sonographers take specific measurements of the fetal head. These include the head circumference (HC) and the biparietal diameter (BPD), which is the distance between the two sides of the head. These measurements are then plotted on specialized prenatal growth charts to determine if they fall below the expected range for the fetus’s gestational age.
Microcephaly can be a progressive condition, meaning head size may appear normal earlier in pregnancy and only become noticeably small later in gestation. For this reason, the condition is often not reliably detected until the late second or early third trimester, around 18 to 22 weeks and beyond. If initial measurements raise suspicion, a follow-up ultrasound is usually recommended to assess the growth trajectory and look for other structural findings.
Postnatal Screening and Evaluation
The most common method for screening for microcephaly after birth is the physical measurement of the infant’s head circumference. This simple, non-invasive procedure, known as measuring the occipitofrontal circumference (OFC), is a standard part of newborn physical examinations and subsequent well-child checkups. A flexible tape measure is used to wrap around the infant’s head, passing just above the eyebrows and ears and over the most prominent part of the back of the head.
Accurate technique is paramount, and the measurement is sometimes repeated multiple times, especially before head molding from delivery has resolved. The measured OFC is then immediately plotted on a postnatal growth chart, such as the WHO charts for infants aged 0 to 2 years, to generate a Z-score. The Z-score provides objective confirmation of whether the infant meets the established criteria for microcephaly. Because the condition can develop after birth (acquired microcephaly), the head circumference must be measured serially at every pediatric visit to track the rate of head growth over time.
Confirmatory and Follow-Up Testing
Once microcephaly is identified through measurement, advanced testing is necessary to confirm the diagnosis, evaluate the brain’s internal structure, and investigate underlying causes. Neuroimaging is a primary tool, with Magnetic Resonance Imaging (MRI) generally preferred over Computed Tomography (CT) scans. MRI provides superior detail of the brain’s soft tissues, allowing clinicians to identify specific structural abnormalities, such as simplified gyral patterns, migrational disorders, or an abnormally small corpus callosum.
CT scans remain useful when there is a need to quickly identify calcifications within the brain, which can indicate certain congenital infections like cytomegalovirus or toxoplasmosis. Beyond imaging, a comprehensive evaluation includes neurological assessments and developmental screenings to evaluate the infant’s motor skills, reflexes, and overall function. Genetic testing, such as chromosomal microarray analysis, and specific blood or urine tests are also performed to investigate infectious, metabolic, or genetic syndromes.