Objective: To determine if the sex ratio of live births in the United States has changed during the 27 years from 1969 through 1995.
Design: Regression analysis of secular trends in sex ratios.
Setting: Population-based data.
Patient(s): Liveborn infants in the United States 1969-1995.
Main outcome measure(s): Sex of liveborn infant.
Result(s): The sex ratio (number of male births divided by number of female births) declined significantly among whites during the 27 years under study. Among black newborns, the sex ratio significantly increased during the same time period.
Conclusion(s): These secular trends could not be explained by changing maternal or paternal age, or by changing proportions of specific birth orders. Possible explanations for the observed changes in sex ratio include random fluctuations in sex ratio over time, changes in demographic characteristics of the population (other than the characteristics controlled for in this analysis), and changes in frequency or timing of intercourse. Environmental exposures are unlikely to account for the observed trends.