Prevention of chromosomal anomalies

Abstract

Extensive newborn chromosome surveys have revealed a frequency of about 5 to 8 abnormalities per 1000 cases studied. These figures do not, however, reflect the true frequency of chromosomal anomalies, as a striking decrease in the frequency occurs from the very early days of the pregnancy through the neonatal period. Almost all the chromosomal anomalies impair intellectual development. This explains why most western populations agree with the principle of prenatal detection.

Prenatal diagnosis of chromosomal anomalies started in the early seventies, with amniocentesis. Ten years later, choriocentesis was made available. Fetal karyotyping is an accurate method with a very low rate of false positive and false negative results. At the beginning of the nineties, what is the impact of these methods on the prevention of chromosomal anomalies?

In south-eastern France, between 1984 and 1988, 22% of trisomies 21 were terminated after prenatal diagnosis. The rate of amniocentesis performed is 5% among women under 35 years of age, 24% among women between 35 and 37 years, 35% between 38 and 39 years and 56% in women over 40 years of age. Obviously, prevention is only partly due to systematic amniocentesis among women over 37. Around 20% of trisomies 21 are diagnosed after detection of growth retardation or a malformation upon sonographic examination, leading to an amniocentesis or fetal blood sampling.

Why do only 55–60% of women over 37 years of age benefit from an amniocentesis, although this technique has been available for nearly 15 years and is performed free of charge for eligible women? Physician's attitudes and knowledge have been shown to be the most important factors for the adoption of this technology. The sociocultural status of women with access to amniocentesis is significantly higher than that of the general population; only systematic screening would be effective in reducing sociocultural inequities in access to prenatal diagnosis. But such a policy may conflict with the respect of women's individual autonomy.

What does the future hold? Several screening tests are being developed which will define new risk groups. Maternal serum AFP, hCG and unconjugated estriol levels, combined with matemal age, can be very useful in determining a combined risk. This should increase the current detection rate to 60% with no additional amniocentesis performed.

New cytogenetic techniques will soon be available. That will allow for quicker results, at a lower cost and at a very early stage of pregnancy.

Chromosomal anomalies are likely to be the first birth defects to be almost completely prenatally screened among medically monitored pregnancies in the coming years.