All good information that you've already received... but here's a piece of information that you may want to ponder between now and the 17th...
No screening is without risk, which compounds the risk of problems, rather than alleviates them. There is nothing that Downs syndrome needs prior to birth that makes knowing 'better' for (unlike congenital heart defects, which can be treated before birth), so there is no real advantage to having the test results (which may or may not be accurate).
The *real* reason for the early screening is to enable you to abort an imperfect child. There are a lot of ethical issues surrounding this -- as the earlier the invasive screening is done, the more dire the consequences can be (smaller babies move more unpredictably, more rapidly and further than bigger, older babies, and can be more easily be injured and killed accidentally by the needle).
So... if you are not willing to have the abortion, the screening need not be done now. Or, frankly, ever.
I happen to be pro-choice, and believe very strongly in a woman's right to her own body... but if you wouldn't abort your baby because of this, think very carefully about the risks you are selecting.