Population studies in northern Sweden. IX. Incidence of Down's syndrome by time, region and maternal age

The incidence of Down's syndrome was studied among 163,043 live-born infants from the two northernmost counties of Sweden, Västerbotten and Norrbotten. The incidence of Down's syndrome, for the period 1954–1974, was 1.28 per mille, which is rather close to the incidence of 1.32 per mille found by LINDSJÖ for total Sweden in 1968–70. The mean maternal age showed a significant decrease with time in both counties. Among mothers of infants with Down's syndrome, a significant change towards lowered maternal age was observed in Västerbotten county, and the same tendency, although not statistically significant, was seen also in Norrbotten. In Västerbotten county, a significant increase in the incidence of Down's syndrome with time was found among mothers below 35 years of age. In one out of 23 regions studied, a significant increase (P < 0.005) in the incidence of Down's syndrome was found.