Recent trends in the prevalence of Down syndrome in Japan, 1980-1997.

The aims of the present study were to determine recent trends in the prevalence of Down syndrome (DS) in Japan, and to determine whether recent changes in demographic and social habits and access to prenatal diagnosis have influenced the livebirth rates of DS. Livebirth statistics indicate that the birth rate in Japan has decreased for women in their 20s and has increased for those in their 30s and 40s. During an 18-year period between 1980 and 1997, 1,299 consecutive DS infants were born among a total of 2,232,694 births, a rate corresponding to approximately 10% of all births in Japan over the same period. The increasing risk of DS with advancing maternal age was confirmed. The overall prevalence was 5.82 DS births per 10,000 livebirths (8.3-9.7 per 10,000 after correction according to the estimated ascertainment ratio: 60-70%). The prevalence rate by year of child birth represents a statistically significant increase (P = 0.001). In conclusion, recent trends in the prevalence of DS in Japan from 1980 to 1997 failed to show a consistent tendency to decrease, probably because of the concomitant increase in pregnancy in advanced maternal age.