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Objective To compare the effectiveness of first trimester, second trimester, and integrated screening for Down's syndrome. Setting Two prenatal diagnosis centres in China. Methods A total of 11,966 pregnant women (≥ 18 years) were screened over 21 months. First trimester screening (11-13 weeks) comprised measurement of serum free beta-human chorionic gonadotrophin (β-hCG) and pregnancy-associated protein-A concentrations, and fetal nuchal translucency thickness. Second trimester screening (15-20 weeks) comprised measurement of β-hCG and alpha fetoprotein concentrations. Computer software was used to calculate the risk of carrying a Down's syndrome fetus. Results The overall incidence of Down's syndrome was 0.2% (23/11,966). When the false-positive rate was fixed at 5%, detection rates for first trimester, second trimester, and integrated screening were 73.9%, 69.6%, and 82.6%, respectively. When the false-positive rate was fixed at 3%, detection/ sensitivity rates for first trimester, second trimester, and integrated screening were 65.2%, 56.5%, and 73.9%, respectively. Conclusions These findings suggest that integrated screening was the most effective means of screening for Down's syndrome in a Chinese population. [ABSTRACT FROM AUTHOR]