Effect of constant (5%) versus gradient (8%-2%) oxygen concentration on sibling human blastocyst development

Does a gradient of oxygen concentration, decreasing from 8% to 2% during prolonged embryo culture, yield a higher percentage of morphologically optimal blastocysts compared with a static 5% oxygen atmosphere?

This prospective sibling-split study included 658 cumulus-oocyte complexes from 44 intracytoplasmic sperm injection (ICSI) cycles (January 2022 to January 2023), alternately assigned to a control group (static 5% oxygen) or an intervention group (stepwise oxygen reduction: 8% on days 0-3, 5% on day 3, and 2% until days 5 or 6).

Early embryo morphology was similar between groups. By day 5, following exposure to 2% oxygen from day 3, the control group produced a significantly higher proportion of clinically used blastocysts (47.3%, 122/258 vs. 36.9%, 97/263; P = 0.0161). Rates of morphologically optimal day-5 blastocysts were comparable (14.7%, 38/258 vs. 10.3%, 27/263; P = 0.2373). Time-lapse analysis showed delayed initiation of blastulation (P = 0.0024), blastocyst formation (P < 0.001), and expansion (P < 0.001) in the intervention group. Because of this delay, embryologists were more likely to select blastocysts for fresh transfer from the control group in a blinded procedure (P = 0.0088). Morphometric analysis showed significantly larger blastocyst cross-sectional area in the control group (28,198 ± 4139 vs. 25,994 ± 4657 µm²; P = 0.0014), consistent with more advanced expansion under constant 5% oxygen.

Gradually lowering oxygen from 8% to 2% during the second half of embryo culture resulted in delayed blastulation and fewer clinically used blastocysts when compared with culture under static 5% oxygen.