Trisomy 21 and its prenatal detection in the Canton of Vaud (1980-1996)

We present a genetic and epidemiological study of trisomy 21 (T21) in the C anton of Vaud, the area severed by our local registry of congenital anomali es which has participated in EUROCAT Switzerland since 1988. During the per iod 1980-1996, we found 240 new T21 cases, all cytogenetically proven, out of 115 064 consecutive live births. Our purpose was to study trends and impact of biochemical screening and pre natal diagnosis of T21. We considered two different peri ods: 1980-1989 (be fore biochemical screening) and 1990-1996 (with screening) during which the mean maternal ages were respectively 28.4 years (10.6% greater than or equ al to 35) and 29.2 years (12.9% greater than or equal to 35). The total prevalence of T21 was 2.08 per 1000; 5.4% of the cases were still births, 49.6% were induced abortions and 45% livebirths. Prenatal cytogenet ic diagnosis of trisomy 21 was performed in 52.1% of cases. Among women age d 35 or over the prenatal detection rates are superposable in the two perio ds. However, for younger women this rate has been much higher since the int roduction of biochemical screening, i.e. 9.8% before and 51.8% after the in troduction of triple test. In conclusion, the increase in prenatal diagnosis tests performed because o f abnormal maternal serum marker levels has increased the global prenatal d etection rate from 36.6% to 63.3% in our population, and the prevalence of Down syndrome has thus slightly decreased among livebirths.