Better quality and more usable embryos obtained on day 3 cultured in 5% than 20% oxygen: A controlled and randomized study using the sibling oocytes

To evaluate the effect of atmospheric oxygen (O₂) concentration on embryonic development, a controlled and randomized study using the sibling oocytes was carried out. A total of 147 patients were studied. Embryos were cultured in O₂ concentration 20% versus 5% during the gamete, zygote, and first 3 days. The mean cell numbers of embryo (7.69 ± 1.91 vs 7.20 ± 1.82, P =.011) and rate of clinically useable embryos (81.62% vs 77.22%, P =.017) were significantly higher in 5% O₂ than in 20% O₂. There was no difference in the zygote developmental stage, day 2, day 4, and blastocyst stage. The quality of blastocyst (both inner cell mass and trophectoderm) showed no difference. Also, there was no increase in embryos fragmentation and uneven cells in 20% O₂ culture condition. In conclusion, 20% O₂ reduced the mean cell numbers of embryo and the number of clinically useable embryos on day 3. However, there was no subsequent negative impact on development of day 4 and blastocyst stage.