Sonographic pseudoasymmetry of the prenatal cerebral hemispheres.

The ultrasonographic image of the fetal head at the appropriate level for determination of the biparietal diameter reveals an apparent asymmetry of the cerebral hemispheres. This was confirmed and analyzed in 49 sequential fetuses who had no neurologic deficits at birth and in the autopsy specimen of an 8-month-old infant without any anatomic abnormalities of the head. In the hemisphere farthest from the transducer, a crescent-shaped echo-poor region was visualized abutting the calvarium. This resembles an abnormal fluid collection but represents normal anatomy. High-amplitude reflections were observed on the medial side of the crescent and were produced by the combination of the sylvian fissure and the choroid plexus of the lateral ventricle. The fine echo pattern of the near hemisphere, which appeared to represent normal anatomy, was artifactual. The cerebral asymmetry seen by conventional ultrasonographic imaging after the 15th gestational week should not suggest an underlying pathologic process. The studies suggest that a higher-quality examination of the near hemisphere of the fetal cranium can be performed with properly focused transducers.