Effects of antenatal diagnosis and selective abortion on frequencies of genetic disorders.

The total public health impact on the frequency of chromosomal disorders using maternal age cut-offs for amniocentesis is relatively small even if all pregnant women beyond 35 years of age were to have amniocentesis. Present-day reporductive practices would only permit the detection of about 20 per cent of cases of Down's syndrome in this age group. Methods to detect chromosomally abnormal fetal cells in maternal blood have much promise for the identification of all women carrying fetuses with abnormal chromosomes. Intrauterine diagnosis of most autosomal dominant disorders is currently not possible. Only relatively frequent autosomal recessive diseases for which simple techniques of heterozygote detection and fetal diagnosis of affected homozygotes are available can be significantly reduced in frequency by intrauterine diagnosis and selective abortion. Only Tay-Sachs disease currently meets these specifications. The paradoxical effect of increasing the frequency of the gene responsible for a disorder such as Tay-Sachs disease following abortion of affected fetuses is discussed but is considered negligible for many generations. Antenatal diagnosis of neural tube defects by assay of amniotic fluid alpha-fetoprotein when carried out following birth of an affected infant in the mother or in an immediate family member has only a small impact on the frequency of this condition. Blood screening for alpha-fetoprotein followed by confirmatory tests potentially can detect a large fraction of affected infants but many logistical problems of false positive and negative results remain. Reduction in the frequency of other multifactorial birth defects by the intrauterine diagnostic approach will require new methods based on blood screening of pregnant women. While the total present impact of antenatal diagnosis on the population frequency of all genetic disorders and birth defects is modest, the usefulness of the procedure in preventing various genetic diseases in families with previously affected members is great and should not be underestimated.