Arachnoid cysts are rarely found during the prenatal period and can exist in any part of the brain as extra-axial cysts. These cysts are usually found after the second trimester and should be differentiated from other types of brain cysts and tumors using ultrasonography and magnetic resonance imaging. Serial sonographic examinations are necessary to evaluate associated central nervous system (CNS) or extra-CNS anomalies and changes in size or shape during pregnancy. If there are other associated anomalies, prenatal genetic evaluations are strongly recommended. Surgical procedures are necessary after birth in approximately 30-60% of the patients. Most isolated cysts have favorable neurodevelopmental outcomes, although information on their prognosis is limited.