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One hundred sixty-four patients with Down syndrome (DS) were confirmed in Tottori Prefecture, Japan, from 1980 to 1999. The sex ratio of 1.52 (99 males and 65 females) was comparable to that reported in previous studies. The live birth prevalence per 1,000 was 1.52 (95% CI: 1.29-1.75) from 1980 to 1999, with a prevalence of 1.34 (95% CI: 1.05-1.63) recorded between 1980 and 1989, and 1.74 (95% CI: 1.37-2.11) between 1990 and 1999. There was no statistically significant change between these two decades (chi(2)-test). Live birth prevalence in these two decades showed a significant increase (chi(2)-test, P < 0.005) compared with that recorded in 1969-1978 in Tottori Prefecture (0.803, 95% CI: 0.677-0.929). Mean ages of mothers at the birth of a DS patient were 31.0 years in 1980-1989 and 32.4 years in 1990-1999 (t-test, no significant difference). Dispersion analysis on the mean age of mothers at birth for patients born between 1969-1978, 1980-1989, and 1990-1999 showed a significant difference (t-test, P < 0.005), while comparing the mean age of mothers in 1969-1978 to those in 1990-1999 also revealed a significant difference (t-test, P < 0.001). Live birth prevalence has increased due to the rise in fertility rates among older women, although maternal age-specific risk rates remain unchanged. The widespread introduction of induced abortion following prenatal diagnosis decreased live birth prevalence of DS largely in European (and a few Asian) countries after 1990, or kept prevalence steady, despite increasing fertility rates among women aged 30 and over. In contrast, all published studies have reported an increase in live birth prevalence of this syndrome in Japan, probably resulting from the fact that prenatal diagnoses are used only exceptionally in this country (due to the negative attitude toward selection of life in Japanese culture).

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