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Last Edited: 11/24/2008

Sex Ratio at Birth

What is the sex ratio at birth?

Sex ratio is most frequently calculated as the number of males in a group divided by the number of females.  It is known that, during pregnancy, males are more likely to be stillborn than females,1 so the sex ratio at conception is slightly higher than it is at birth.  The sex ratio at adulthood in a population may again be different from that at birth if survival rates between males and females differ during childhood.

For most times and places, the human sex ratio at birth is thought to stay in the neighborhood of 1.05; that is, there are slightly more males at birth than there are females.  One analysis of 250 years of data in Finland2 suggested that this ratio rose between 1750 and the first part of the 20th-century but has declined since.  In California, this ratio has declined over the second half of the 20th-century, although this pattern varied (and sometimes ran in the opposite direction) depending upon the race and ethnicity of the parents.3


1.    Davis D, Gottlieb M, Stampnitzky J. Reduced ratio of male to female births in several industrial countries. Journal of the American Medical Association. 1998;279(13):1018-1023.
2.    Vartiainen T, Kartovaara L, Tuomisto J. Environmental chemicals and changes in sex ratio:  analysis over 250 years in Finland. Environmental Health Perspectives. 1999;107:813-815.
3.    Smith D, Von Behren J. Trends in the sex ratio of California births, 1960-1996. Journal of Epidemiology and Community Health. 2005;59:1047-1053.