Objective: In all continents, latitude gradients and secular trends have been found in the male-female ratio at birth (M/F: male births divided by total births), which is anticipated to approximate 0.515.
Methods: Annual national data for countries comprising Europe and North America for male and female live births were obtained from the World Health Organisation and analysed with contingency tables.
Results: This study analysed 397278548 live births. An overall decreasing trend in M/F was found (p < 0.0001). A latitude gradient was also noted, with more males being born in southern, warmer latitudes in Europe. The converse occurred in North America, with more males born in northern latitudes (p < 0.0001). There was an overall deficit of 2053687 male births based on an anticipated M/F of approximately 0.515. The declining M/F trend has reversed over 2000-9 in Mexico stabilised in the United States. M/F in the North American continent was unaffected following the Chernobyl event of 1986. There was a significant rise in M/F in Central Europe only in 1987 returning to the previous baseline over the following two years.
Conclusion: M/F is decreasing in Europe and North America, in contrast with the rising M/F previously found in Asia, South America and the post-Soviet states. The North American M/F latitude gradient is similar to that found in South America, with increasing M/F in higher and colder latitudes. Europe’s latitude gradient is similar to that of Asia’s, with more males born closer to the equator. No facile explanation/s for these findings are apparent.
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