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

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.