Microcephaly is a condition where a baby’s head is smaller than expected compared to other babies of the same age and sex. During pregnancy, medical professionals use ultrasound as a primary diagnostic tool to monitor fetal development. This imaging technique allows for the careful observation of fetal growth, which includes tracking the size of the head, helping to identify developmental variations early on.
The Role of Ultrasound in Prenatal Screening
The anatomy scan, performed between 18 and 22 weeks of gestation, is a detailed examination where numerous measurements are taken. It is often during this scan that concerns about conditions like microcephaly may first arise. The purpose of this mid-pregnancy ultrasound is to evaluate the baby’s anatomy and compare its growth against established milestones. If measurements fall outside the expected range, it can signal the need for closer monitoring or further diagnostic testing.
Key Measurements on an Ultrasound
When assessing fetal head size, sonographers focus on several specific biometric measurements. A primary measurement is the Head Circumference (HC), which is determined by tracing an ellipse around the outer edge of the fetal skull. This measurement gives a total size of the head and is a fundamental indicator of growth.
Another measurement is the Biparietal Diameter (BPD), which measures the distance between the two parietal bones across the widest part of the head. This is complemented by the Occipitofrontal Diameter (OFD), measuring the length of the skull from the forehead to the back of the head. These linear measurements provide additional data points about the shape and dimensions of the head.
These collected measurements are plotted on standardized growth charts specific to gestational age. A diagnosis of microcephaly is considered when the HC falls more than two standard deviations below the average, or below the 3rd percentile. In some cases, a more severe classification is used if the measurement is more than three standard deviations below the mean.
Interpreting Microcephaly on Ultrasound Images
Beyond quantitative data, clinicians look for visual cues on the ultrasound that can suggest microcephaly. The overall shape of the head is assessed, with a sloping or receding forehead sometimes being an observable characteristic.
An important aspect of interpretation is proportionality. The head measurements are compared against other fetal biometrics, such as the abdominal circumference (AC) and femur length (FL). A head that is disproportionately small when compared to a body that is otherwise growing normally is a significant indicator that helps distinguish isolated microcephaly from overall fetal growth restriction, where the entire body is smaller than expected.
The ultrasound can also reveal other associated features within the brain that may point to an underlying cause. The presence of intracranial calcifications, which are small calcium deposits in the brain tissue, can be a sign of congenital infections like cytomegalovirus (CMV) or Zika virus. The ultrasound might also show a simplified pattern of brain folds, a condition known as lissencephaly, which can be linked to microcephaly.
Next Steps After an Abnormal Ultrasound
An ultrasound indicating potential microcephaly is the beginning of a more detailed diagnostic process. The initial finding leads to a referral to a maternal-fetal medicine (MFM) specialist, who focuses on high-risk pregnancies. This specialist will often recommend a more advanced, high-resolution ultrasound, sometimes called a level II scan, to get a more thorough view of the fetal anatomy.
For an even more detailed image of the fetal brain, a fetal MRI may be suggested. This imaging technique uses a magnetic field instead of sound waves and can provide a clearer picture of the brain’s structure. A fetal MRI is useful for examining the complex folds and structures of the brain that may not be fully visible on an ultrasound.
To investigate potential genetic causes or infections, a procedure called amniocentesis may be offered. This involves taking a small sample of the amniotic fluid for testing, as the fluid contains fetal cells that can be analyzed. Alongside these tests, genetic counseling is provided to help parents understand the potential causes, prognosis, and what to expect.