Screening for fetal chromosome aberrations in early pregnancy.

Seven years' experience in prenatal screening for fetal chromosome aberrations in the west of Scotland is reviewed. Fetal chromosome analysis was undertaken in 716 pregnancies, 49% of which were judged to be at substantial risk of a fetal chromosome aberration. A chromosome aberration was found in 26 pregnancies, 14 of which were sufficiently severe to justify termination: the latter included nine cases of trisomic Down's syndrome, two cases of translocation, two cases of XXY Klinefelter's syndrome and one case of the triple-X syndrome. Ten pregnancies with balanced fetal chromosomal translocations and two with extra, small metacentric chromosomes of no clinical significance continued normally in pregnancy. Prenatal diagnosis permitted many mothers at risk the opportunity of having a family which otherwise they would not have attempted, and saved a number of pregnancies which would have been terminated solely on the risk, rather than the diagnosis, of fetal abnormality. An unexpectedly high frequency (6-6%) of severe fetal chromosome aberrations was found in pregnancies of mothers aged 40 years and over. In the maternal age groups 35-39 years the frequency was 1-4%. It is concluded that specialized cytogenetic facilities are urgently required to allow older mothers the option of prenatal screening. This is also required for younger mothers who have previously had a child with Down's syndrome, and for families at risk of chromosomal translocation and X-linked disease. Prenatal screening is best provided on a regional basis by a department of medical genetics experienced in genetic counselling, human cytogenetics, and cell culture techniques, working in close collaboration with obstetrical colleagues and the ultrasound department.