【Objective】 To investigate the effect of incubator oxygen concentration on the embryo development and outcome of women undergoing ICSI and embryo transfer. 【Methods】 A total of 645 ICSI cycles with fresh embryo transfer conducted in the First Affiliated Hospital of Sun Yat-sen University during January 2011 to December 2011 were retrospectively analyzed. According to the oxygen concentration during day 1-3 in vitro culture

cycles were classified as 5% oxygen concentration group and 20% oxygen concentration group. Day 3(D3) transfer embryo score and clinical outcomes were measured. 【Result】 Viable embryo rate (72.02% versus 68.73) and mean score of transfer embryo (22.53 versus 20.10) in D3 culture were higher in low oxygen group while all clinical outcomes were similar. Cumulative pregnancy rate (60.44% versus 51.67%) was higher in lower oxygen group. To subgroup cycles with poor prognosis

mean score of transfer embryo (20.66 versus 16.71)

viable embryo rate (76.11% versus 64.55%) and implantation rate (22.27% versus 10.08%) was significantly higher in low oxygen group. 【Conclusion】 In vitro culture of embryos deriving from ICSI in 5% oxygen concentration during the first 3 days is superior to culture in 20% oxygen concentration.