CHANGING SEX-RATIO IN THE UNITED-STATES, 1969-1995

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: Populat ion-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. A mong black newborns, the sex ratio significantly increased during the same time period. Conclusion(s): These secular trends could not be exp lained by changing maternal or paternal age, or by changing proportion s of specific birth orders. Possible explanations for the observed cha nges in sex ratio include random fluctuations in sex ratio over time, changes in demographic characteristics of the population (other than t he characteristics controlled for in this analysis), and changes in fr equency or timing of intercourse. Environmental exposures are unlikely to account for the observed trends. (Fertil Steril(R) 1998;70:270-3. (C)1998 by American Society for Reproductive Medicine.).