Objectives: To investigate whether the time of embryo transfer (ET) affect the oocyte-to-baby rate. Materials and Methods: The database was retrospectively analyzed including total number of oocytes collected and corresponding oocyte-to-live baby born (LBB) rate. Then the relationship between different time of embryo transfer and oocyte-to-baby rate was compared. In a year period all patients undergoing infertility treatment were included in the study. The outcome parameters were total number of oocytes collected and corresponding oocyte-to-LBB. Results: For patients under the age of 35 years, there was no increase in oocyte-to-LBB regardless of the time of ET. For patients older than 35 years, the oocyte use rate increased significantly when embryo was transferred on day 2. Oocyte-tobaby rates were also analyzed after grouping patients on the number of oocytes retrieved per cycle. For patients < 35 years, the oocyteto- LBB rate increased significantly on day 3 if < ten oocytes were obtained. whereas for patients > 35 years, the oocyte-to-baby rate was best on day 2 when about 15 oocytes were retrieved. Conclusions: This retrospective analysis demonstrated the relationship between the time of ET and ooctye-to-baby rate that is indicative of a more biologically efficient reproductive system.