The effect of two distinct levels of oxygen concentration on embryo development in a sibling oocyte study

Purpose: This prospective randomized study used sibling oocytes of 258 women with ≥8 oocytes to compare the effect of 5 % O₂ versus 20 % O₂ concentrations on embryo development and clinical outcome. Methods: Oocytes of each case were divided between incubators with either 5 % or 20 % O₂ concentration. Outcome measures were fertilization, cleavage, embryo quality, blastocyst formation, and implantation, pregnancy and live birth rates. Results: Fertilization and cleavage rates were similar in both groups. The 5 % O₂ group had significantly more blastomeres (P < 0.05) and more top-quality embryos on day 3 (P < 0.02), as well as significantly more available embryos for transfer (31.6 % vs. 23.1 % for the 20 % O₂ group; P < 0.0001). There were significantly more cycles with good embryos in the 5 % group (76/258) than in the 20 % group (38/258) (P < 0.0001). Implantation and pregnancy rates were significantly higher for 5 % O₂ embryos (P < 0.03 and P < 0.05, respectively). Live birth rates per embryo transfer were 34.2 % and 15.8 %, respectively, P < 0.05. Conclusions: Implantation, pregnancy and live birth rates are higher, and more good quality embryos are available for transfer and freezing with reduced rather than with atmospheric oxygen concentrations during embryo incubation.