Lateral ventricles - 9.3 mm at 20 weeks U/S

Hello,
Just came back from my 20 weeks U/S - and the both lateral ventricles were measured at the top of 10mm upper range limit (9.3 mm).
I was told to reschedule an U/S in three weeks, to see what happens.
What is the average size? How dangerous is it? What are my options in case the ventricles will keep getting larger?
Everything else was normal, and in place...
Thank you for your help and time!