Biochemical Screening for Down’s Syndrome

Abstract

Until the early 1980s the only effective method of identifying women with a risk of having a fetus with Down’s syndrome sufficient to justify a diagnostic amniocentesis was to use maternal age; older women, say 40 years of age or more, were offered an amniocentesis whilst younger women were not. Unfortunately, there is considerable overlap in the distribution of maternal age in pregnancies with and without Down’s syndrome and most affected pregnancies occur in young women. Using an age cut-off level of 40 years, about 1% of women are eligible for an amniocentesis and this would lead to the detection of about 15% of pregnancies associated with Down’s syndrome. By lowering the cut-off to 35 the detection rate is increased to about 35% but with an increase in the amniocentesis rate to about 7%. Offering amniocentesis to younger women who have had a previous Down’s syndrome pregnancy is accepted practice, but it has only a negligible effect on the overall detection rate; about 1% of affected pregnancies will arise in such women. In practice, few screening programmes based on maternal age and previous family history have achieved a reduction in the birth prevalence of Down’s syndrome of more than 15%.