Seasonal variation in secondary sex ratio in Edo State, Nigeria: a retrospective study

ABSTRACT

Season is known to influence human secondary sex ratio but information on this subject is lacking in Nigeria. The objective of this study was to determine the secondary sex ratio (SSR) during the wet and dry seasons in Nigeria. In this retrospective cohort study, the records of all deliveries at St Philomena Catholic Hospital (SPCH), Benin City, Edo State, Nigeria between 1st January, 2005 and 31st December, 2014 (10 years) were retrieved and analyzed. The births were recorded according to the year and month of delivery. Stillbirths and infants with ambiguous genitalia were excluded from the analyses. The total number of live-births during the 10-year period under review was 13,702 and this consisted of 7,007 males and 6,695 females, resulting in a secondary sex ratio of 104.6100 (1.051). In general, the monthly distribution of births was bimodal with a greater peak in May and a lesser peak in October with the highest and lowest SSRs in the months of June and March, respectively. In the dry season, the proportion of male births was higher than the proportion of female births; p > 0.05. In contrast, in the wet season the proportion of male births was lower than the proportion of female births; p > 0.05. In addition, the SSR was higher in the dry season (1.04) compared with the wet season (0.99). In Edo State, Nigeria, the proportion of male births tended to be higher than the proportion of female births during the dry seasons.