Oscar
Both blood test and scan. The scan is also called the NT test (nuclar transluncency) to measure the thickness of back of neck bone (okok I'm not medical professional, but as far as I can rem from sat two preg.). I think the thickness should not exceed 2.5mm or something...
Based on your age there is a 'risk' like 1 in 1000, the scan and blood test will give a more accurate like 1 in 10 000 chance etc...
It's all risk factor/percentage/number...
Amnio
The invasive procedure to draw amniotic fluid from sac which of course means a risk of miscarriage. This is usually done if you have higher than normal chance from your scans.
Detailed scan at 20 weeks
Important because there are other things to pick up (gender, limb length, cleft palate, growth, heart, kidney, brain etc etc...) abnormal growth..
That's asuch as I rem...disclaimer: my memory fails me at times, please do not hold me accountable for the info. As mentioned the views given above do not come from a healthcare professional. Better ask your gynae for more info..