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Abstract
It is common knowledge that more males than females are born and that the sex ratio at birth usually varies between 104 and 107. Since Graunt first observed a disparity between the number of male and female births in 1662, many researchers have investigated the levels, patterns of variation, and determinants of the sex ratio of live births. Some further patterns of variation in the sex ratio at birth are described in this study, and the extensive body of literature on the determinants of the sex ratio at birth is discussed. Finally, the question of the relationship between hepatitis B and the sex ratio at birth is examined.
Race is the most striking factor influencing the sex ratio at birth: black populations have consistently lower sex ratios at birth than white populations. In Europe, there seems to be a gradient in the ratio, the lowest values being concentrated in the Center-West. In some countries (Sweden, England and Wales, Japan), secular upward trends are observed.
Some evidence of the effect of individual characteristics on the sex ratio at birth has been cited in recent literature. Parents of higher socioeconomic status seem more likely to bear sons than parents of lower socioeconomic status. Births to younger fathers and births of lower orders seem to have a higher chance of being male. One hypothesis suggests that the sex ratio at birth is a direct reflection of the sex ratio at conception and is determined by the levels of maternal hormones circulating at that time. The present research indicates that the main factors seem to operate in the early, rather than the late, stages of pregnancy.
Recently, altered sex ratios at birth have been observed in the years following epidemic outbreaks of certain infectious diseases. The relationship between hepatitis B infection in parents and the sex ratio at birth of their offspring is examined here. This study suggests that the hepatitis B carrier state in parents may increase the sex ratio of their offspring. The available data are too scarce for a definite answer, but this study does not support the hypothesis that immunization against hepatitis B would greatly affect the sex ratio at birth of the future generations.
