Canadian researchers carried out computer simulations of the cost-effectiveness and outcomes for 19 potential screening strategies on 110,948 pregnancies.
A range of screening models are currently in use, and some may be leading to women being inappropriately deemed to be at high-risk.
Fewer miscarriages and unnecessary terminations would occur if women were classed in the first trimester as high-risk when there was a one in nine chance of the fetus having Down's syndrome, the researchers found.
High-risk women would then be offered early diagnosis by chorionic villous sampling. Low-risk women would be reassured and not undergo second trimester testing.
The researchers warn, however, that risk levels are often misunderstood by patients.
'It may be difficult to refuse a diagnostic test at first trimester for an anxious woman with a calculated risk close to the cut-off because some women want an early diagnosis,' they say.
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