Hi Nicky, at the 20 week u/s my boy twin was found to have a choroid plexus cyst with an enlarged left ventricle. We had to have a target scan done the next week, nothing was found to be worried about, and Alexander was born without any problems at all, and we were never told that he had to have a head scan.
If I remember correctly, it happens in up to 3% of all pg's, but google left brain ventricle ultrasound, and you should ge the info you want.
Hugs to you, as it is so hard not to worry about things when your pg, but remember, if it was something serious, your doctor would tell you straight away.
If you want to know anything else, don't hesitate to ask.