Genetic aspects of prenatal diagnosis.

With improved control of environmental agents, genetic conditions are now a major cause of residual handicap and mortality in all age groups. Primary prevention of this diverse group of over 5000 distinct disorders is not yet possible and effective therapy is, as yet, available for very few. Hence, the present emphasis on prevention is directed towards the identification and testing of pregnancies at risk in order to allow the option of early selective termination of pregnancy. Although over 400 distinct conditions have already been successfully prenatally diagnosed, the identification of at-risk pregnancies on the basis of a positive family history alone can identify only a minority of affected pregnancies. In contrast, genetic screening during (or before) pregnancy offers a real prospect for detection of a majority of affected pregnancies and hence for reducing the birth frequency of serious genetic conditions. This is exemplified by the impact of the maternal serum alpha-fetoprotein screening programme for neural tube defects and is under active development for autosomal aneuploidies and certain other major congenital malformations. These screening programmes will result in a reduced frequency of mental and physical handicap in the community but their implementation will require a comprehensive team approach with supraregional (or national) funding and co-ordination.