Prevalence of Down's syndrome

The impact of legal termination of pregnancy and of prenatal diagnosis on the birth prevalence of Down syndrome in Denmark

A considerable reduction in number of livebirths for mothers over 35 was observed in Denmark from 1960 to 1980. Birthrates for those aged 35-39 fell by 58.8%, for those aged 40-44 by 78%. In 1979-1980 100 infants with Down syndrome were born among 116757 newborns, a birth prevalence of 0.86 per 1000, which was significantly lower than the incidence of 1.17 per 1000 when the prenatally diagnosed cases were included. The reduction was noticeable for the age group over 35 where it fell to 1.89 per 1000 for mothers 35-39 and 6.48 per 1000 for mothers over 40. The utilization of prenatal diagnosis was 72 per 100 livebirths for women 35 and older in the Copenhagen area and 56 per 100 livebirths for the rest of the country, with differences in different areas. The number of induced abortions for women 35 years and older was 9265 against 6597 livebirths. The estimated number of Down syndrome cases averted by unrestricted abortion was 61, twice the number prevented by amniocentesis (31), with the greatest impact for mothers over 40. An increased risk of Down syndrome for the age group 35-39 was observed when liveborn and prenatal cases were considered together showing an incidence of 6.89 per 1000, with the highest incidence in the Copenhagen area, 8.70 per 1000, more than double the incidence of 3.04 observed in Copenhagen from 1960 to 1971, for the same age group.

Down's syndrome in Western Australia: cytogenetics and incidence

235 cases of Down's syndrome were ascertained in a 10-year study of Down's syndrome in Western Australia. Although cytogenetic studies performed on 222 subjects confirmed that 95% of cases were trisomic due to nondisjunction, 4% were trisomic due to translocation, and 1% were mosaic, the ratio of inherited/sporadic translocations differed from that usually reported. Comparison of the results with those of an earlier Australian survey of Down's syndrome demonstrated a real fall in the incidence of Down's syndrome in Australia but no significant change in maternal age-specific incidences.

Factors affecting the incidence of Down syndrome in Scotland

The incidence of Down syndrome in Scotland between 1960 and 1974 was estimated using data from thirteen Area Health Boards. Incidences in individual years, in different months of the year and in different maternal age groups were also determined. There was an apparent increase in incidence in more recent years almost certainly due to increased ascertainment. This may have obscured any decrease in incidence which might be expected as a result of a decrease in mean maternal age during this period and the introduction of antenatal diagnosis. No seasonal differences in incidence could be detected either in the whole area or in individual Area Health Boards. There was an increase in incidence with maternal age which was not significantly different from that found by other investigators. There was also an indication of an increase in relative incidence (% Down syndrome births/% live births) over the period 1966–74 in mothers aged 35 years and older.