Arachnoid cyst is accumulation of cerebrospinal fluid within the arachnoid membrane due to maldevelopment of the arachnoid or secondary to trauma or infection. Neonatal arachnoid cyst is rare, representing 1% of intracranial masses, with 50-60% occurring in the middle cranial fossa. The prognosis is generally good. However, neonatal morbidity and mortality depend on the location of the cyst and complications, such as acute mass effect by intracystic hemorrhage or the development of a subdural hygroma or hematoma. Therefore, early diagnosis and treatment is critical. A diagnostic character of prenatal ultrasound imaging is well capsulated homogeneous hypoechoic cyst in normal arachnoid cisterns. Nevertheless, US has the limitation of differential diagnosis from other brain cysts. Fetal MRI is the excellent procedure in the aspect of its ability to demonstrate the exact location, extent, and relationship of the arachnoid cyst to adjacent brain or spinal cord. We present two cases of arachnoid cysts diagnosed by prenatal ultrasound. In one
Case with bilateral large cyst, fetal MRI confirmed arachnoid cysts. A female infant is alive and well with shunt operation, developing normally on long-term follow-up (2 years). In another case, postnatal MRI demonstrated an arachnoid cyst. A male infant was also good and asymptomatic after birth.