Background: The expected ratio of male to female births is generally believed to be 1.05, also described as the male proportion of 0.515.
Objectives: We describe trends in sex ratio at birth and in fetal deaths in the United States, in African Americans and in whites, and in Japan, two industrial countries with well-characterized health data infrastructures, and we speculate about possible explanations.
Methods: Public health records from national statistical agencies were assembled to create information on sex ratio at birth and in fetal deaths in the United States (1970-2002) and Japan (1970-1999), using SPSS.
Results: Sex ratio at birth has declined significantly in Japan and in U.S. whites, but not for African Americans, for whom sex ratio remains significantly lower than that of whites. The male proportion of fetal death has increased overall in Japan and in the United States.
Conclusions: Sex ratio declines are equivalent to a shift from male to female births of 135,000 white males in the United States and 127,000 males in Japan. Known and hypothesized risk factors for reduced sex ratio at birth and in fetal deaths cannot account fully for recent trends or racial or national differences. Whether avoidable environmental or other factors--such as widespread exposure to metalloestrogens or other known or suspected endocrine-disrupting materials, changes in parental age, obesity, assisted reproduction, or nutrition--may account for some of these patterns is a matter that merits serious concern.