3 Chromosome analysis on chorionic villi

Abstract

Chromosome abnormalities are a major health problem. About 25% of mentally retarded individuals have a chromosome aberration. Very little is known about the causes of non-disjunction giving rise to trisomies, the largest group of chromosomally abnormal individuals. As primary prevention is not possible methods have been developed to study fetal chromosomes with the aim of aborting affected fetuses. Most of the studies have involved screening of women 35 years and older--they have also allowed couples with translocations and other aberrations, who would otherwise fear to reproduce, to achieve a normal family. Hitherto, one of the greatest problems of fetal diagnosis has been the timing of the test in the second trimester of pregnancy with the risk of having to terminate a wanted pregnancy around the twentieth week of gestation. The first trimester approach would overcome this problem. PREVALENCE OF CHROMOSOME ABERRATIONS Chromosome aberrations can be diagnosed by cytogenetic analysis of metaphases from lymphocyte cultures or fibroblast cultures from skin or other tissues, amniotic fluid cell cultures or recently by a direct method or cultures from chorionic villi. Chromosome abnormalities are found with an average incidence of 6 per 1000 live births. The majority of chromosome aberrations at birth are aneuploidies due to a non-disjunctional event. The birth prevalence of viable trisomies rises with advanced maternal age. About 0.3% are chromosomal rearrangements, most of them balanced without phenotypical effect. Only a minority have unbalanced rearrangements with more or less severe developmental abnormalities. Inherited translocations have a high risk of unbalanced offspring depending on the type of rearrangement and in certain cases on the sex of the carrier parent. In sporadic aberrations the recurrence risk is about 1% (Stene and Mikkelsen, 1984). A much higher prevalence of chromosome aberrations was observed in spontaneous abortions (Hassold et al, 1980). Also, studies of perinatal deaths show a higher prevalence of chromosome aberrations compared to live birth figures (Kuleshov, 1976).