Oh Clare - what a terrible time you are having.
Think of it this way. There is every chance that you will haver a perfectly healthy and happy little baby. Every chance!!!!!
By introducing the risk associated with CVS at this stage, the drs are really looking after your best interests by potentially saving your baby from trauma. You dont want to be the stat that has problems after such a procedure. Having been through a m/c, I wouldn't wish it on anyone. The guilt may do your head in.
By waiting 3 weeks the drs will have more confidence that they can carry out the procedure without the added risk, and thats got to be the best way to go for you.
Your OB may not have any more answers for you than that. While its a science, mother nature has an awful lot to do with things, and the best OB in the world can't control life.
It sounds like your baby is growing, so enjoy your pregnancy as best you can and hold tight.
Big hugs for you - this all seems like a bad dream I'm sure.