The effect of induced abortion on the incidence of Down's syndrome in Hawaii

Abstract

There was a decrease in the recorded number of cases and in the incidence rate of Down's syndrome in Hawaii between 1963-1969 and 1971-1977. Independent of all other factors, induced abortion accounted for 43 percent of the decline in the number of cases, based on the assumption that a substantial number of clandestine abortions were being performed in Hawaii before the 1970 legalization of abortion. However, if we assume that very few illegal abortions were performed prior to 1970, there would have been an actual 3.5 percent increase in the number of cases of Down's syndrome in the absence of legal abortion. Declining pregnancy rates and decreasing age-specific incidence rates of Down's syndrome also contributed to the drop in the number of cases between 1963-1969 and 1971-1977.

PIP: This study uses data from Hawaii birth and death certificates and from the Department of Health Mental Retardation Registry for 1963-69 and 1971-77 to investigate whether there has been a decrease in the number of babies born with Down's syndrome since the legalization of abortion, and to what degree such a decrease may be attributed to legal abortion. High and low estimates of Down's syndrome prevalence and 3 types of measures used to estimate the change in its incidence lead to the conclusion that the number of cases and the incidence rates of Down's syndrome fell by between 24.5% and 48% between 1963-69 and 1971-77. A mathematical decomposition was made of the relative impact on the decline in number of cases observed between the 2 periods of 5 factors: Down's syndrome incidence rates by age, number and distribution of women of childbearing age, and pregnancy, abortion and fetal death rates by age. Four sets of data are derived, the most likely of which assumes that a significant number of induced abortions occurred before legalization and that the recorded cases of Down's syndrome represent almost complete coverage. Under these assumptions the number of cases of Down's syndrome fell by 24 between the 2 periods, and 43% of the total decline was accounted for by a rise in the rate of induced abortions, independent of all other factors. Declining pregnancy rates and decreasing age-specific incidence rates also contributed to the decline in number of cases.