Secular trends and latitude gradients in sex ratio at birth in Asia during the past 60 years.

Background: Latitude gradients and secular trends in Europe and North America 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 Asian countries for male and female live births were obtained from the World Health Organization and analyzed with contingency tables.

Results: A total of 245,938,211 live births were analyzed. An overall increasing trend in M/F was found (P < 0.0001). A latitude gradient was also noted, with more boys being born in southern, warmer latitudes (P < 0.0001). There was an overall deficit of 1,351,757 male births based on an anticipated M/F of approximately 0.515.

Conclusion: M/F is increasing overall in Asia, unlike the decline previously noted in Europe and North America. Moreover, it had been shown that there is a higher incidence of male births in southern Europe than in the north, with the opposite gradient in North America. This paper shows that M/F latitude gradients in Asia are in keeping with those of Europe. The overall M/F in Asia may be rising due to improving socioeconomic conditions, and the interplay of several poorly understood factors is likely.