Gigi
no you are half right CVS (done around 12wks) and Amnio (after 15-18wks or so) are different - they are two invasive tests - though they are guided by ultrasounds where a needle is inserted into uterus and sample is taken -they are not down routinely even in older woman unless clinincaly indicated or the mother/OB wants it done.

The Nuchal translucency is non invasive ultrasound ( you also have additional bloodtest ) -the scan measures the fluid at the back of the baby's neck - if this is high and combined with blood test results.

They give you an age adjusted risk factor - estimating the chance of down symdrome is 1/? (anywhere from 18 to 21,000 ).The higher the risk the lower the number - 1/66 is high risk 1/1200 -low .Especially if under 1/300 they will refer and recommend you have CVS or wait for amniocentisis -but optional 1/300 means 299 chances the baby is normal.

The 12 wks scan is also used for a basic check of structural abnormalities like skull (complete) and limbs - dates the pregnancy too if you are unsure.The 18-20 wk is a much more through anatomy scan.