I have just returned from my 12 week scan, and I was lucky enough to see an experienced obstetrician. It is really worth paying for this if you can.
I have experienced periodic bleeding to date, a few times heavy and bright red, and he was able to show me a second egg sac belonging to a lost twin that I miscarried at the start. Apparently losing a twin is much more common than is realised, and the second site often bleeds without causing any risk to the viable remaining twin. It is a huge relief to know that although I will probably have a few more bleeds before it shrinks away, they will not be anything to panic about. Many sonographers miss this, so if you have unexplained bleeding you may be in the same boat.
He also had the time and experience to be able to confirm I did not need a blood test or CVS test for Down's, based on measurements alone. I am 39, and would be automatically be assessed as high risk under the standard procedure. I am very relieved, as I was worried about the miscarriage risk from CVS.