Nuchal scan they just check nuchal fold at back of baby neck. They then take bloods an using your age and some other information, they give you a ratio which determines whether you are high or low risk to be carrying a baby with DS.
If they give you a ratio lower than 1:250 they see you as a higher risk. Anything above 1:250 is not high risk. Depending on your dates, if they think you a high risk they will offer you a CVS (which carries a 2% or less chance of mc) or an amnio (which carries a 1% or lower chance of mc).
Pro of a CVS is that it is earlier done than an amnio which gives you more time to
Make a decision if the result comes back that baby does have chromosomal abnormalities!
If this does happen though, believe me, as a woman who is carrying a baby with DS, it really is not the worst thing in the world. It's something that you get use to very easily when you get the news!
To be honest though, you are far more likely to not be carrying a baby with abnormalities. So, if you go for the nuchal scan, try not to worry too much!
Good luck xxx