Secular trends in sex ratios at birth in North America and Europe over the second half of the 20th century.

CONTEXT A previous study showed that significantly more boys were born in southern latitudes in Europe than in northern latitudes and the converse pattern was observed in North America. OBJECTIVE This study analyses secular trends in gender ratios for live births over the second half of the 20th century. DESIGN, SETTING, PARTICIPANTS Analysis was carried out from a World Health Organisation dataset comprising live births over the above period. This included 127034732 North American and 157947117 European live births. MAIN OUTCOME MEASURES Analysis of trends in gender ratios for countries in both continents. RESULTS The findings show a highly significant overall decline in male births in both Europe and North America (p<0.0001), particularly in Mexico (p<0.0001). Interestingly, in Europe, male births declined in North European countries (latitude>40 degrees, p<0.0001) while rising in Mediterranean countries (latitude congruent with 35-40 degrees, p<0.0001). These trends produced an overall European male live birth deficit 238693 and a North American deficit of 954714 (total male live birth deficit 1193407). CONCLUSIONS No reasonable explanation/s for the observed trends have been identified and the causes for these trends may well be multifactorial.