Screening for Down's syndrome using an iso-risk curve based on maternal age and serum alpha-fetoprotein level.

On the basis of the significantly different distributions of maternal serum alpha-fetoprotein (AFP) levels in 86 pregnancies associated with fetal Down's syndrome and in 2018 unaffected pregnancies, an iso-risk curve for Down's syndrome was constructed. An iso-risk curve shows, for women of all ages, which combinations of maternal age and level of maternal serum AFP result in the same risk of carrying a fetus with Down's syndrome. A 1:400 risk of Down's syndrome, corresponding to the risk of a 35-year-old woman, was chosen as the lowest risk indicating referral for amniocentesis. If all women, irrespective of their age, are offered amniocentesis, when their risk of carrying a Down's syndrome fetus is 1:400 or higher, 53% of the affected fetuses can be detected as compared with 28% of the affected fetuses diagnosed at present in women above 35 years of age.