Influence of demographic factors on annual Down's syndrome births in Ohio, 1970-1979, and the United States, 1920-1979.

Because of the high correlation between advanced maternal age and the risk of Down's syndrome, the demographic factors of population age structure and age-specific birth rates are expected to have considerable effect on Down's syndrome births. The principal objective of this research was to clarify the past and present role of dramatic changes in these two demographic factors within the United States in determining the number and incidence of Down's syndrome births, and the percentage of these births to women greater than or equal to 35. The main hypothesis is that demographic events, modified by increasing use of amniocentesis, adequately account for the general trends regarding Down's syndrome births reported in the recent literature, as opposed to other explanations such as environmental factors or a real biologic increase in Down's syndrome births in younger women. The hypothesis was tested using estimates of the annual number of Down's syndrome births in Ohio for 1970-1979, obtained through use of the Lincoln-Peterson Capture-Recapture method of estimating population size. These were compared to predicted N derived from Ohio demographic data and single year maternal age Down's syndrome risk rates from Upstate New York. The two values of N were in statistical agreement in 8 of the 10 years compared, and show an overall incidence of Down's syndrome in Ohio during the 1970s of 1.20. Using USA government demographic data and Ohio 1970-1979 single year maternal age Down's syndrome risk rates, predicted Down's syndrome births were derived for the USA during 1920-1979. The results show a long-term decline over the period in predicted number (6870 to 4232), incidence (2.42 to 1.23), and percentage of Down's syndrome births to women greater than or equal to 35 (64.3 to 23.4). A precipitous decline in all three values was obtained for the 1960s and early 1970s, with an abrupt end to the declines during the mid 1970s. Predicted number of Down's syndrome births rose 10.4% during the last half of the decade from a low of 3835 in 1975, and is expected to continue to rise rapidly throughout the 1980s, principally as a result of the large increase in the number of women moving into the older age categories.