Impact of oxygen concentrations on fertilization, cleavage, implantation, and pregnancy rates of in vitro generated human embryos.

The aim of the present study was to determine the impact of oxygen concentration during in vitro culture of human oocytes and embryos on fertilization, cleavage, implantation, pregnancy, multiple gestation and abortion rates. Women 20-48 years old presenting for infertility treatment and accounting for 3484 in vitro fertilization/intracytoplasmic sperm injection cycles were included in the study. Oocytes/embryos were randomly allocated to be incubated under three different oxygen tension environments: (1) 20% O2 in air; (2) initially 20% O2 in air, followed on day 2 (2-4 cells stage) by 5% CO2, 5% O2 and 90% N2; and (3) 5% CO2, 5% O2 and 90% N2 throughout. Interestingly, IVF-derived embryos cultured in 5% O2 yielded higher rates of fertilization and implantation as compared to those incubated in 20% O2 (P < 0.05). Conversely, embryos in 20% O2 yielded higher rates of fertilization, high quality embryo and implantation than those in the 20%-5% O2 group (P < 0.05). Moreover, ICSI-derived embryos cultured in 20% O2 resulted in lower rates of cleavage as compared to those from the 20%-5% O2 group (P < 0.05). These results are consistent with in vitro and subsequent in vivo embryo development being more susceptible to O2 tension fluctuations rather than the degree of O2 tension itself during culture.