This study evaluated whether the concentration of biphasic O2 (5-2%) promotes the formation of qualified blastocysts (QBs) and euploid blastocysts and the probability of cycles with transferable blastocysts. The paired experimental design included a total 90 patients (180 cycles) without euploid blastocysts in previous monophasic O2 (5%) cycles were enrolled for an additional cycle of biphasic O2 (5-2%). In the biphasic O2 (5-2%) group, the QB rate (35.8%, 225/628) was significantly higher than that in the monophasic O2 (5%) group (23.5%, 137/582; p < 0.001). In addition, the euploid blastocyst number (0.5 ± 0.8) and the percentage of cycles with transferable blastocysts were significantly higher in the biphasic O2 (5-2%) group (57.8%, 52/90) than those in the monophasic O2 (5%) group (0 and 35.6%, 32/90, respectively; p < 0.01). Multivariable regression analysis also indicated that the QB rate and the probability of cycles with transferable blastocysts correlated with O2 tension (OR 1.535, 95% CI 1.325-1.777, and OR 3.191, 95% CI 1.638-5.679, respectively; p < 0.001). Biphasic O2 culture can be used as an alternative strategy to increase the euploid QBs and the probability of cycles with transferable blastocysts in patients with a poor prognosis.
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