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.