Prospective study of prenatal screening for Down's syndrome with free beta human chorionic gonadotrophin.

Abstract

OBJECTIVE--To assess the value and impact of a screening programme for Down's syndrome that uses the two maternal serum markers: alpha fetoprotein and free beta human chorionic gonadotrophin. DESIGN--All women booked into clinics were screened. Further tests were offered to women with a risk of one in 300 or greater of having an affected baby. Follow up of outcome of all pregnancies. SETTING--Biochemical screening laboratory serving two health districts. SUBJECTS--8179 women of all ages with singleton pregnancies screened between 15 and 22 weeks' gestation from 1 April 1991 to 31 March 1992. MAIN OUTCOME MEASURES--Detection rate of Down's syndrome, false positive rate, uptake of screening, uptake of amniocentesis in women identified as at increased risk, prevalence of Down's syndrome at birth. RESULTS--Overall 89% (8317/9345) of women underwent screening. The rate of detection of Down's syndrome was 69% (11/16; 95% confidence interval 41 to 89%) with a 5.2% false positive rate (426/179; 4.7 to 5.7%). In women under 30 the detection rate was 50% (four out of eight; 32 to 86%) Uptake of amniocentesis was 89% (389/437), resulting in a reduction of prevalence of Down's syndrome at birth from 1.1 per 1000 in previous years (66/59,696) to 0.4 per 1000 during the screening year (4/9345). Additionally, several other abnormalities were identified. CONCLUSION--The benefit of a high detection rate with this approach and the additional anomalies identified should encourage others to introduce screening programmes for Down's syndrome that use free beta human chorionic gonadotrophin and alpha fetoprotein.