Is it a choroid plexus cyst? If so, my second son had one they found at his anatomy scan and it was gone at 28 weeks. The doc said sometimes it’s just a pocket of fluid build up on the brain and they disappear as their brain develops more. Try not to stress!
My baby has one too they said its because there brain is still developing they usually go away about 27 weeks...praying all goes well with your little bean!!🙏🏻🙏🏻