Abstract： Objective To investigate the status of second trimester maternal serum screening for Down's syndrome from 2006 to 2009 to evaluate the screening quality in Beijing.Methods Birth defect data (birth defect register) from 2006 to 2009 and screening data reported by certificated screening hospitals from 2008 to 2009 in Beijing were retrospectively analyzed by descriptive statistic method.Results Three hundred and thirty Down' syndrome babies were delivered from 2006 to 2009 in Beijing,among which 166 mothers accepted Down' syndrome serum screening and 85(51.2％) were diagnosed finally according to the screening results;among the 85 mothers,35 were≥35 years old with the detection rate of 62.5％ (35/56),and 50 cases were ＜35 with the detection rate of 45.5％ (50/110).The prenatal diagnostic rate of Down's syndrome babies increased from 26.8％(11/41) in 2006 to 47.4％(46/97) in 2009.The detection rate of Down's syndrome with serum screening increased from 23.1％(3/13) in 2006 to 62.9％(22/35) in 2009.One hundred and fifty-eight 21-trisomy cases accepted mid-term ultrasound screening,among which 62 cases (39.2％) were found abnormal.Conclusions Quality control for second trimester maternal serum screening for Down's syndrome should be improved in Beijing.Second trimester maternal serum screening for Down's syndrome in elderly primipara is not suggested.More attention should be paid on prenatal ultrasound screening.