Trends in the sex ratio of California births, 1960-1996

Study objective: The male sex ratio at birth (or the proportion of male births in a population) has been suggested as a sentinel environmental health indicator. Usually around 51%, the proportion may be dramatically decreased in offspring of persons with chemical exposures. Recent publications from the USA and elsewhere have noted a small but apparently declining male birth proportion, suggesting the effect of some environmental exposures. This paper sought to examine these trends more closely in California's large and diverse population. Design: Using computerised birth certificate data, time trends were examined by multivariate linear and spline regression, controlling for demographic factors. Setting: California. Participants: About 15 million births from 1960 to 1996. Main results: In the raw data, the male birth proportion is indeed declining. However, during this period, there were also shifts in demographics that influence the sex ratio. Controlling for birth order, parents' age, and race/ethnicity, different trends emerged. White births (which account for over 80%) continued to show a statistically significant decline, while other racial groups showed non-statistically significant declines (Japanese, Native American, other), little or no change (black), or an increase (Chinese). Finally, when the white births were dividied into Hispanic and non-Hispanic (possible since 1982), it was found that both white subgroups suggest an increase in male births. Conclusion: This analysis shows that the decline in male births in California is largely attributable to changes in demographics.