The human sex ratio at birth (SRB) and frequency of twinning are demographic parameters that vary among populations. A retrospective study was carried out to investigate the trend of SRB, as well as twinning rate in Ibadan, Nigeria. Data on sexes of singletons, twins, triplets and quadruplets from 1997 to 2008 collected from the University College Hospital (UCH), Ibadan were analyzed by year, quarter, month and maternal age-groups using descriptive tabulation time series analysis, Chi-square and student t-test. An upward trend was observed in the SRB with an average ration of 110.1:100 for the period of study. The highest and lowest sex ratio of 126.6:100 and 50:100 were recorded in the 15-19 and <15 years of maternal age-groups respectively. The mean frequency of twin births was 35.9‰ with the highest of 52.7% in 1997 and lowest of 27.8% in 2006. The maternal age- group 20-24-year had the highest occurrence (46.5%) of twin births, while the lowest (21.8‰) was recorded in the 15-19-year age-group. Trend analysis shows a downward trend in the frequency of twin births, but still ranks among the highest values in the world. It is believed that maternal age, genetic and some socio-environmental factors might have influenced the SRB and twinning rate. These findings are consistent with earlier studies in Nigeria.