I have experienced similar to you Dani. We had our 20 weeks scan and they said the measurement was 10mm then 9mm and then 9.5. They referred us to Kings College Fetal Medicine unit because they got 10 (they refer when 10 and above and as they got one measurement of 10 they played it safe). We were really upset and were referred that day. Kings, the best place in the country, were amazing and their knowledge and equipment was spectacular. They got a measurement of 8.5 and then 8.9mm. We were told that it was most likely it would be, and a quote, 'a variant of normal with good prognosis'. As the baby had no other problems (and they checked everything) and my Downs risk at 12 weeks was low, with Nuchal measurement of 1.7. Also, as our baby is a boy this is another indicator as they have larger heads with more fluid on average and so it's more common.
We were told by Kings to come back at 28 weeks to check the measurement. If it has gone up then they will do a fetal MRI. However, they emphasised that they were over 90% sure it would be okay.
Now, I am rubbish at waiting so I have booked an appointment to see Professor Nikolaides at the Fetal Medicine Centre in Harley St in three weeks when I will be 25 wks to check on the measurement. He is the most experienced man in the country and even come up with the nuchal measurement etc, so if he says it's all okay I will be very happy.
I don't know if this helps. Ventriculomegaly is not necessarily most likely to lead to hydrocephalus in my opinion and I have conducted a lot of research (I have a PhD so I am very much into my research).
I think there are forums on the i-am-pregnant website that may help you
Good luck. Try not to google as it can make it worse. I would try to get as much info about your individual case from the docs.