Patient: At 20 weeks of mid pregnancy, it shows widening of the left lateral ventricle and enlargement of the ipsilateral choroid plexus compared to the opposite side of the fetus. A: The ventricles are cavities in the brain and there are four of them: a pair of lateral ventricles and one each in the third and fourth ventricles. They are connected to each other. The cerebrospinal fluid is mainly generated by the lateral ventricles and the choroid plexus of the third ventricle, then through the third ventricle and the middle cerebral canal to the fourth ventricle, and then through the median and lateral foramina of the fourth ventricle into the subarachnoid space. The main function of the cerebrospinal fluid is to protect the brain and spinal cord from shock, and to supply o brain nutrition and output metabolites through the perivascular space. The width of the fetal lateral ventricle is measured by ultrasound in China, and the width of the posterior horn of the lateral ventricle is usually measured on the side of the fetal thalamus.