3RD VENTRICLE - SIZE AND APPEARANCE IN NORMAL FETUSES THROUGH GESTATION

PURPOSE: To define the size and appearance of the normal fetal third v entricle. MATERIALS AND METHODS: The third ventricle was prospectively assessed in 441 consecutive normal second- and third-trimester fetuse s. The fetuses were divided into six gestational age ranges. Data rega rding the size and configuration of the third ventricle were analyzed for each group. RESULTS: The third ventricle was seen in 435 of 440 (9 8.9%) fetuses. It appeared as a single echogenic line between the thal ami in 171 (38.9%) fetuses, as parallel echogenic lines outlining a fl uid-filled lumen in 243 (55.2%) fetuses, and as divergent lines deline ating a V-shaped fluid-filled structure in 21 (4.8%) fetuses. The sing le-line configuration was most common early in the second trimester. L ater in pregnancy, the ventricle walls could be discerned as separate parallel or divergent lines outlining a fluid-filled lumen. The averag e width of the ventricle was relatively constant at approximately 1 mm from 12 to 28 weeks. After this time, it enlarged, reaching a maximum 1.9 mm. CONCLUSION: The third ventricle can be imaged in most second- and third-trimester fetuses. Its size and configuration evolve throug h the second and third trimesters. This evolution must be considered i n the evaluation of normality. At any gestational age, a third ventric le greater than 3.5 mm in width should be viewed with concern for abno rmality.