We opted out because of personal reasons but there are 'soft markers' for DS a sonographer can pick up in an ultrasound. The thing is just because baby may have one of these soft markers does mean they might have DS, its just an indication that it might be worth investigating further such as with a follow up amniothesis screening.
*doesn't mean they have DS
I didn't do it either because people told me that a lot of the time the tests come back positive even when they're not. To do more testing they pull some of your placenta out with a needle. It's very dangerous to the baby...
The amniothesis screening, where they insert the needle to collect some amniotic fluid, gives you a percentage rather then a yes or no. So it really just adds more information if baby already has some or one of the 'soft markers' e.g. lesions in the liver, renal pelvic dialation.
They measure the fluid at the back of babys head and it just says weather thes a risk or not low risk your baby doesn't have have a chance of down syndrome if its high then they will I think take a bit of fluid from around the baby from belly button they will just run test but have you had the scan 1?
They have a new blood test that actually is safer then the needle in the belly (sorry not sure what it is callled) and 100% safe compared to the risk that other injection has.