Trends and seasonal variations in human secondary sex ratio in Southwest Nigeria: A 10-year survey

Introduction: Secondary sex ratio is the number of live male births per 100 female births. In humans, the primary sex ratio, which is the sex ratio at conception, is expected to be 1:1 in natural populations based on the law of independent assortment of the X and Y chromosomes. This is not the case with the sex ratio at birth as it is largely affected by various social and cultural practices that it tends to bias towards one sex, with profound implications for population and demographic dynamics. Materials and Methods: We collected retrospective data of records of live births from three Southwestern Nigerian states, covering 10 years. The data were obtained from different Hospitals/Birth Centres from three states, i.e., Lagos, Ondo and Osun, between 2006 and 2017. Data analysis was performed to determine the monthly, quarterly, yearly and seasonal variation in sex ratios. Chi-square analysis was used to determine the significance of differences in sex ratios at P < 0.05. Results: Annual average sex ratios of 99.7, 105.6 and 106.0 were obtained for Lagos, Osun and Ondo, respectively, while the pooled data had a sex ratio of 104.2, indicating male preponderance. Sex ratios differ significantly according to season. Differences in sex ratio were significant during the dry season in Lagos and Osun states and in Ondo during the rainy season. Conclusions: Sex ratios from Southwest Nigeria are as diverse as other populations, with ratios ranging between 99.7 and 106.0.