Context: The sex ratio of 1.06:1, the ratio of male to female births, has declined over the past decades. Recent reports from a number of industrialized countries indicate that the proportion of males born has significantly decreased, while some male reproductive tract disorders have increased.
Objectives: To examine the evidence for declines in the male proportion at birth and suspected causes for this decline, and to determine whether altered sex ratio can be considered a sentinel health event.
Data sources: Birth records were analyzed from national statistical agencies.
Study selection: Published analyses of trends in ratio of males to females at birth and studies of sex determinants evaluating epidemiological and endocrinological factors.
Data extraction: Proportion of males born: 1950-1994 in Denmark; 1950-1994 in the Netherlands; 1970-1990 in Canada; and 1970-1990 in the United States.
Data synthesis: Since 1950, significant declines in the proportion of males born have been reported in Denmark and the Netherlands. Similar declines have been reported for Canada and the United States since 1970 and parallel declines also have occurred in Sweden, Germany, Norway, and Finland. In Denmark, the proportion of males declined from 0.515 in 1950 to 0.513 in 1994. In the Netherlands, the proportion of males declined from 0.516 in 1950 to 0.513 in 1994. Similar declines in the proportion of males born in Canada and the United States are equivalent to a shift from male to female births of 8600 and 38000 births, respectively. Known and hypothesized risk factors for reduced sex ratio at birth cannot fully account for recent trends.
Conclusion: Patterns of reduced sex ratio need to be carefully assessed to determine whether they are occurring more generally, whether temporal or spatial variations are evident, and whether they constitute a sentinel health event.