A transverse cerebellar diameter of 27mm at 23 weeks gestation is usually in the normal range. The transverse cerebellar diameter is the horizontal length of the cerebellum, which can be used to indirectly assess the weeks of the fetus. The transverse cerebellar diameter is categorized into longitudinal diameter, transverse diameter and thickness, which together describe the three indicators. Normally the transverse cerebellar diameter is used to check whether the fetus is healthy or not. If the test value is below the normal range, the fetus is considered to have possible cerebellar atrophy. If it is greater than the normal range, hydrocephalus may also be possible. The cerebellar transverse diameter is related to the age of the fetus, and a transverse diameter of 27 mm at 23 weeks of gestation is considered normal. If the transverse cerebellar diameter is not normal during pregnancy, amniocentesis can be performed to check for malformations or possible maternal abnormalities, and the cause of the abnormality should be identified in a timely manner.