Relative growth of the higher fetal brain structures.

In a retrospective review of 90 normal obstetric ultrasonographic examinations obtained between 15 and 35 menstrual weeks, measurements were made of the transverse dimensions of the thalamus, basal ganglia/insula, atrium of the lateral ventricle, and temporal operculum in the standard biparietal diameter plane. At 15 menstrual weeks the echogenic choroid plexus, filling the lateral ventricle, dominates the appearance of the cranium. During later gestation, the cerebral hemispheres and basal ganglia (telencephalic structures) show marked growth, while the diencephalon and lateral ventricles increase much less rapidly in size. This observation is made using a method of measurement that least favors the demonstration of cerebral cortical growth. Of note is that the width of the lateral ventricle remains relatively stable throughout the second and third trimesters.