A left-sided subventricular cyst of 10 mm in a newborn is mostly a benign lesion and is not very serious. Subventricular cysts on the left side of newborns are usually due to a developmental abnormality that occurs during the congenital development of the fetus due to various factors. Most of these anomalies do not affect the growth and mental development of the newborn. However, it is important to review the condition regularly and monitor the child carefully for any abnormal symptoms, such as abnormal muscle tone. If the cyst does not go away, but instead grows larger, it is important to pay attention to it, which requires neonatology consultation and, if necessary, surgery.