If ultrasound shows a high risk of Down syndrome, many women opt for amniocentesis, which can give a definite diagnosis. This does run a (small) risk of inducing a miscarriage though.
I think this knowledge is out of date; the cell-free DNA test I talked about is both more reliable (it sequences the genome, and Down's is an easily-detectable chromosomal disorder) and is available earlier than an ultrasound or amnio.
