none of the general scans risk miscarriage, they 12 week downs test just measures the fluid at the baby's neck and gives a risk factor based on this. If you are considered high risk you will be offered a further test, such as the amnio where a fine needle withdraws some of the amniotic fluid from around the baby and this is tested. I believe this test has a 1% risk of miscarriage.

It depends where you are but when I had my kids in Geelong the 12 and 19 weeks scans were done in the hospital and fully covered by medicare. Any early scans due to early bleeding I had were at my own cost.