Objective: To assess the secondary sex ratio (SSR) of assisted reproduction technology babies within a fertility clinic in Melboe, Australia. Methods: A retrospective cohort study was conducted on 3369 babies who were b following single embryo transfer between 2011 and 2016. Variables examined included embryo creation date, matal and patal ages, matal body mass index (BMI), type of infertility, sperm parameters, fertilization method, type of embryo and stage of transfer. Multivariate regression analysis was performed on the data set using STRATA V9.2. Results: More males were found to be b to embryos created in summer (adjusted odds ratio=1.46, 95％ confidence interval: 1.08-1.95; P=0.01). Lower BMI of mother (≤30 kg/m 2) and younger age of parents (≤35 years) were associated with an increase in SSR (50.1％ vs. 47.1％; 50.6％ vs. 48.8％; 50.0％ vs. 48.9％) respectively. Decreased SSR value was associated with primary subfertility, using in-vitro fertilization procedure and fresh embryo compared to those with secondary subfertility, using intracytoplasmic sperm injection procedure and thawed embryo (49.3％ vs. 50.6％; 47.1％ vs. 50.8％; 49.4％ vs. 50.1％) respectively. Conclusions: The fertilization methods, type of embryo, stage of embryo transfer, parameters of the sperm and status of subfertility do not significantly affect the SSR. SSR is affected by the seasonality, matal age, and BMI.