meegm my DD's was about the size of a pea when she was born and it was on the back of her skull. She had it regularly checked (about every 6 months) until she was 18 months old, then they did an MRI to determine if it had "roots" and if it was above or in the skull. She also had xrays done. Through that process, they determined it wasn't attached, and was most likely a cyst sitting on top of the skull but not attached to it. She was booked in for surgery and had that shortly after she turned 2 (day surgery). They sent the cyst off to the lab to confirm it was nothing nasty.

The cyst did grow from the size of a pea when she was born, to the size of a marble by the time she had it removed.

The specialist didn't say it was or wasn't rare.... just didn't come up I guess.