Declining sex ratios in Canada.

OBJECTIVE To examine the trends in the proportion of annual live births that were male in Canada and to compare the trends with those in the United States. DESIGN Analysis of census data. SETTING Canada as a whole and 4 main regions (West, Ontario, Quebec and Atlantic). SUBJECTS All live births from 1930 to 1990. OUTCOME MEASURES Sex ratio (expressed as the proportion of total live births that were male [male proportion]) overall and by region. RESULTS The male proportion in Canada decreased significantly after 1970 (p < 0.001); this represented a cumulative loss of 2.2 male births per 1000 live births from 1970 to 1990. Although a decrease was observed in all four regions studied, only that in the Atlantic region was significant (p < 0.001), representing a cumulative loss of 5.6 male births per 1000 live births from 1970 to 1990. A significant decrease in the male proportion was also observed in the United States from 1970 to 1990 (p < 0.001), although to a lesser degree than that observed in Canada, and represented a cumulative loss of 1.0 male births per 1000 live births. CONCLUSIONS The decreased sex ratio in Canada adds to the growing debate over changes in biological markers and their potential causes. In addition, the study illustrates the potential use of the sex ratio as a widely available, unambiguous measure of the reproductive health of large populations.