[Prenatal study of trisomy 21: triple test more efficient than age criterion]

To compare the results of selection of gravidae for trisomy 21 testing on the basis of age or of the triple test.

Theoretical evaluation.

Demographic statistical data for the years 1980, 1985, 1990 and 1994 and the age-specific risk were used to calculate the expected number of children to be born with trisomy 21. Also calculated were the size of the risk groups, the number of children with trisomy 21 to be detected early, the iatrogenic loss of pregnancy and the number of invasive tests to be performed in order to detect one child with trisomy 21 with either selection method.

Shifting of pregnancy to women with a mean older age results in a increase of the risk group as determined by age. It will also result in an increase of the number of iatrogenic losses of pregnancy. Selection of the risk group by the triple test will not result in these increases. The risk group, as determined by the triple test, includes about 60 per cent of the women pregnant of a child with trisomy 21; that determined by age 23 to 30 per cent. The number of invasive tests to be performed for the detection of one child with trisomy 21 is significantly smaller in case of selection by the triple test.

By application of the triple test less pregnant women have to be further examined and more cases of trisomy 21 are detected, than by application of the age criterium.