Microcephaly is a condition where a baby’s head is significantly smaller than expected for their age and sex. This reduced head size often indicates that the brain has not developed properly during pregnancy or its growth has slowed after birth. Ultrasound imaging during pregnancy serves as a primary tool to screen for and identify potential cases of this condition. It allows healthcare providers to visualize the developing fetus and measure various parameters that can indicate abnormal growth patterns.
Detecting Microcephaly with Ultrasound
Ultrasound examinations during pregnancy are a standard method for assessing fetal growth and development, including head size. Sonographers measure the fetal head circumference (HC) and biparietal diameter (BPD) to estimate overall head size. The HC is a measurement around the widest part of the fetal head, while the BPD measures the diameter across the head from one parietal bone to the other. These measurements are then compared to standardized growth charts that indicate the expected head size for a given gestational age.
A measurement falling below a certain percentile for gestational age can raise suspicion for microcephaly. A head circumference measuring two or more standard deviations below the mean for gestational age, or below the 3rd percentile, is considered concerning. More severe cases are defined as three or more standard deviations below the mean. The presence of a sloping forehead, due to disproportion in the frontal lobes, can also be a visual indicator that increases suspicion.
Beyond simple measurements, sonographers also look for other ultrasound findings that might accompany microcephaly or suggest underlying brain anomalies. These can include intracranial calcifications, small deposits of calcium within the brain tissue, or ventriculomegaly, an enlargement of the fluid-filled spaces (ventricles) within the brain. Simplified gyral patterns, where the normal folds and grooves of the brain appear less complex, may also be observed, indicating abnormal brain development.
Timing and Accuracy of Detection
Microcephaly is identified during the routine second-trimester anatomy scan, performed around 18 to 22 weeks of gestation. While some major anomalies can be detected earlier, microcephaly often becomes more apparent as the pregnancy progresses. This is because in early pregnancy, the fetal head size might still fall within the normal range, even if brain development is not proceeding as expected.
Detecting microcephaly with ultrasound can be challenging, and its accuracy is influenced by several factors. The gestational age at the time of the scan plays a role, as the condition may not be clearly visible until later in pregnancy. Fetal position and the skill of the sonographer also impact the quality and reliability of the measurements. Mild cases of microcephaly are difficult to detect prenatally, and some fetuses initially suspected of microcephaly via ultrasound may have a normal head circumference at birth.
The accuracy of prenatal ultrasound in detecting microcephaly can vary. While a head circumference measurement below the 3rd percentile is a common indicator, ultrasound serves as a screening tool that raises suspicion, prompting further investigation rather than providing a final diagnosis.
Next Steps After Suspected Microcephaly
If microcephaly is suspected based on an ultrasound, several subsequent actions are taken to gather more information and confirm the diagnosis. A more detailed fetal ultrasound, sometimes including neurosonography, is performed to get a closer look at the fetal brain structures. This specialized ultrasound can provide clearer images of brain development and identify any additional findings.
A fetal MRI (Magnetic Resonance Imaging) may be recommended for better visualization of brain structures. MRI can overcome some limitations of ultrasound, offering more detailed images of brain anatomy and detecting abnormalities in neuronal migration. Combining MRI with ultrasound can provide more specific prenatal diagnostic information.
Genetic testing is a significant step to identify underlying causes of microcephaly, which can include genetic mutations, chromosomal abnormalities, or infections. This may involve amniocentesis for chromosomal microarray analysis (CMA) or specific gene panels. CMA can detect extra or missing sections of genetic material linked to the condition. Testing for infections like Zika virus, toxoplasmosis, rubella, or cytomegalovirus (TORCH infections) may be conducted.
Referral to specialists, such as maternal-fetal medicine specialists, genetic counselors, and neurologists, is an important part of the process. These specialists can provide comprehensive evaluations, discuss potential causes, and offer counseling regarding the prognosis and management options. Counseling and support for expectant parents facing this diagnosis are offered to help them understand the condition and prepare for potential outcomes.