Down's syndrome in Wallonia (South Belgium), 1971-1978: cytogenetics and incidence.

During the 8 year-period 1971-1978 inclusive, 268 newborn with Down's syndrome (DS) were ascertained in Wallonia (South Belgium). The chromosomes of all patients were analyzed. A standard trisomy 21 was observed in 259 cases (96.6%) and translocations in seven (2.6%). One mosaic (0.4%) and one case with a 47,XX,+21,5 p-karyotype (0.4%) were also found. The overall incidence at birth for the 8 year-period was of 0.123%. However, the comparison of the 2 four-year periods 1971-1974 and 1975-1978 showed an increased incidence in mothers below 35 during the second period, reaching a statistical significant level in Charleroi, and industrial and densely populated area of the region (Fisher's exact test: P = 0.016 less than 0.025). Young women have now become the principal source of DS patients in Wallonia. This change from the previous pattern happened abruptly in 1975, and strongly suggests that one or more persisting factors relating to meiotic non-disjunction of chromosome 21 are operating in this region since 1974, and affecting DS births from 1975 onwards.