You need to clarify what kind of cyst has been detected. The commonest are choroid plexus cysts that are, by and large, harmless and will resolve. The ultrasonologist must also ensure that there are no other associated anomalies in the baby. If there are no associated problems, there is no cause for alarm and you will need serial ultrasounds to monitor the size of the cysts. When in doubt take a second opinion with a ultrasonologists who specializes in this kind of ultrasounds.