The finding supports the use of non-invasive methods over amniocentesis.
Researchers measured nuchal translucency in pregnant women, between 11 and 14 weeks of gestation, from a sample of 1.1 million Danish inhabitants.
A greater-than-normal amount of fluid, over 6mm, pointed to high risk of Down's syndrome.
A combination of serum markers in the maternal blood test was also found to increase the risk of a chromosomal abnormality.
Chorionic villus sampling was used to confirm or exclude Down's syndrome where test results showed an elevated risk.
The number of children born in Denmark with Down's syndrome was halved as a result.
The findings were presented last week at the annual conference of the European Society of Human Genetics, in Nice.