Annual Variation Of Sex Ratio In Twin Births And In Singletons In Brazil

The annual variation of the sex ratio (SR) of 1385 twin births was analyzed and compared to that of 85909 singletons. These births referred not only to live births but also to stillbirths in two southeastern Brazilian maternity hospitals from 1984 to 1993. While the annual variation of the SR of singletons was very small, that of twin births was extremely high, due to the significant heterogeneity of the annual data. It is suggested that the large SR annual variation of the twin birth might be a consequence of the variation of male or female monozygotic twins. The hypothesis that twin births show a lower sex ratio than singletons could not be supported by the present data.This article presents the findings of a study of the annual variation of the sex ratio among twin births compared to singleton births among a maternity population in two urban hospitals in Brazil. Data are obtained from hospital records from a maternity hospital in Campinas and Sao Paulo, Brazil, during 1984-93. There were 1385 twin births and 85,909 singleton births delivered during 1984-93 in these maternity hospitals in Sao Paulo state. Deliveries included stillbirths weighing 500 g or more. Findings indicate that the sex ratio of singletons was 104.6 males per 100 females. The sex ratio of the 763 twins born in Campinas and the 622 twins born in Sao Paulo showed considerable variation from the mean sex ratio of singleton births. The mean sex ratio among twins in Campinas was 107.1 males per 100 females, with a standard deviation of 20.4 in Campinas and 29.7 in Sao Paulo. There were significant differences from the mean sex ratio among twin births in Campinas in 1984 and 1989 and in Sao Paulo in 1984, 1985, 1988, and 1989. There was significant heterogeneity of sex ratios among twin births even with pooled data. The mean of pooled data on twin births was 102.9 males per 100 females, with significant differences from the mean in 1984, 1985, and 1987. The application of Woolf's test indicated heterogeneity in the data and no effect on the sex ratio from twin births. Maternal age was negatively correlated with the sex ratio of twin births. It is argued that monozygotic twins were not primarily females and that twin births did not have a lower sex ratio than singleton births.443-416316