Is a large head circumference concerning on fetal growth ultrasonography (ultrasound)?

From the Guidelines

A large head circumference on fetal ultrasound is concerning and requires further evaluation, particularly if it measures more than two standard deviations above average for gestational age. This finding could be normal, especially if proportional to overall fetal growth and if parents have naturally larger head sizes. However, it might indicate conditions such as hydrocephalus, macrocephaly, or certain genetic syndromes, as suggested by the international consensus recommendations on the diagnostic work-up for malformations of cortical development 1. The relationship between head circumference and abdominal circumference (HC/AC ratio) is particularly important; a disproportionately large head compared to the abdomen may suggest asymmetric growth restriction or other developmental issues.

When a large head circumference is detected, your healthcare provider will likely recommend additional ultrasounds, possibly a detailed anatomical scan, and in some cases, amniocentesis or MRI for further assessment, as recommended by the society for maternal-fetal medicine consult series on the diagnosis and management of fetal growth restriction 1. The finding should be evaluated in the context of other measurements, family history, and overall fetal development to determine its clinical significance. It is also important to consider the guidelines for fetal monitoring and management in hypertensive disorders of pregnancy, which recommend fetal biometry, amniotic fluid volume assessment, and fetal Doppler waveform analysis 1.

Key considerations in the evaluation of a large head circumference include:

• The gestational age at which the measurement was taken
• The presence of other fetal anomalies or growth restrictions
• The HC/AC ratio and its implications for fetal development
• The need for additional testing, such as amniocentesis or MRI, to determine the underlying cause of the large head circumference
• The importance of ongoing fetal monitoring and management to ensure the best possible outcomes for the fetus and mother.

In terms of management, the American College of Radiology Appropriateness Criteria recommend transabdominal ultrasound as the mainstay of initial management and assessment of fetal growth, with additional testing and monitoring as needed to identify and manage potential complications 1. Ultimately, the clinical significance of a large head circumference on fetal ultrasound requires careful evaluation and consideration of multiple factors to determine the best course of action.

From the Research

Fetal Head Circumference on Ultrasound

• A large head circumference on fetal growth ultrasound can be concerning, as it may indicate macrocephaly, which is defined as a head circumference above the 98th percentile or greater than +2SD above the mean for gestational age 2.
• Macrocephaly can be primary, due to increased brain tissue, or secondary, due to various causes such as hydrocephalus, cerebral edema, or brain tumors 2.
• The accuracy of fetal head circumference measurement on ultrasound can be affected by several factors, including gestational age, low liquor, and anterior placenta location 3.
• A study found that ultrasonographic head circumference underestimated actual postnatal head circumference in 87.5% of cases, and the error increased with advanced gestational age 3.

Implications of Large Head Circumference

• A large fetal head circumference is associated with complicated labor and is a significant risk factor for assisted vaginal births and emergency cesarean sections 4.
• The prevalence of prolonged labor, signs of fetal distress, and maternal distress increases with increasing head circumference 4.
• In cases of suspected macrosomia, patients must be counseled carefully regarding a delivery plan, and cesarean section should be considered when indicated 5.
• Techniques to assess for suspected macrosomia include clinical measurements, ultrasound, and MRI 5.

Measurement and Evaluation

• The growth of fetal head circumference, chest circumference, and abdominal circumference can be assessed using ultrasound measurements 6.
• The correlation between gestational weeks and fetal head circumference is high, with a correlation coefficient of 0.972 6.
• The growth of large-for-date fetuses is more rapid than that of appropriate-for-date fetuses, and it can be difficult to detect large-for-date fetuses before 30 weeks 6.