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Background Case 1: Mum was a 30 years old multiparous woman and known smoker. Antenatal ultrasound scan showed an intracranial arachnoid cyst measuring 14 × 13 mm, that continued to grow in subsequent scans. Baby was born at 37+5 weeks and day 2 cranial ultrasound scan showed a large cystic lesion on the suprasellar region measuring 17 × 17 mm. MRI on day 6 of life showed a large suprasellar arachnoid cyst 18 × 22 mm in size which was causing significant mass effect displacing and compressing the midbrain and basal ganglia. As the baby was clinically asymptomatic and progressing and developing appropriately, the neurosurgical team opted for a conservative management with a multidisciplinary approach with involvement of ophthalmology, endocrinology, and local general paediatricians for close monitoring of growth and development as well as regular blood tests (Renal function and pituitary function). He remains well until the present and is currently 15 months old. Case 2: Mother was 34 years old smoker and multiparous. Antenatal ultrasound scan showed a left sided 25 × 33 mm intracranial arachnoid cyst. Postnatal cranial ultrasound performed on day 3 of life showed a large arachnoid cyst 37 × 47 mm in size on the left posterior area of the brain. At the Neurosurgical follow up clinic MRI showed the cyst was stable in size with mild mass effect noticed. The baby remains under close follow up and monitoring by the neurosurgical team, with conservative management until the present at age of 16 months. Objectives Methods Results Review & Discussion: Arachnoid cysts are benign cysts that occur in the cerebrospinal axis in relation to the arachnoid membrane. They are most commonly developmental anomalies, however some are acquired and they represent 1% of all space occupying lesions in childhood. Arachnoid cysts in the antenatal period can be found in isolation or with other intracranial abnormalities associated. The antenatal diagnosis should be confirmed by neonatal neuroimaging; cranial ultrasonography is the first line imaging modality and should happen within the first 24 hrs of age. Brain MRI is the gold standard investigation and should happen within the first 2 weeks of life. Antenatally diagnosed arachnoid cysts may remain clinically silent in postnatal period or even regress spontaneously; but they usually have a tendency to grow rapidly in the perinatal period. There is no clear guidance on the most appropriate management strategy for these cases. Management include different types of neurosurgical procedures, but endoscopic ventriculocystocisternostomy seems to be the overall preferred approach. Conclusions Arachnoid cysts are rare and can remain silent during childhood before presenting with clinical manifestations. Rapid enlargement of incidentally diagnosed antenatal arachnoid cysts can occur in perinatal period. Serial evaluation by imaging in the perinatal period is necessary to demonstrate the rapid cyst enlargement and the need for appropriate intervention. Neurosurgical approach is variable and dependent on clinical features and the department’s experience or preference, but the cysts which occur in isolation without any other intracranial anomalies and with normal karyotype usually lead to a good outcome.