Abstract:

Objective To compare the effects of sequential culture media and single-step culture media on early embryonic development in IVF/ICSI procedures， and to assess the differences between different single-step culture medium formulations. Methods This study conducted a retrospective analysis of 259 patients who obtained ≥10 retrieved oocytes per cycle between June and December 2025， with a total of 3，578 mature oocytes included. A sibling oocyte study design was adopted to compare early embryo development indicators and Day 3 blastomere fusion between the sequential culture medium group and the single-step culture medium group， where the latter was further divided into 6 subgroups： OS-1 to OS-6. Results No statistically significant differences were observed between the two groups in the 2PN cleavage rate， Day 3 good-quality embryo rate， blastocyst formation rate， usable blastocyst rate， or clinical pregnancy rate （all P > 0.05）. Compared with the sequential culture group， the single-step culture group exhibited a lower high-quality blastocyst rate （31.36% vs. 39.80%， P < 0.001） and a higher Day 3 cell fusion rate （6.69% vs. 3.55%， P < 0.001）. Multivariate regression analysis demonstrated that single-step culture medium was independently associated with decreased odds of obtaining high-quality blastocysts （OR = 0.700， 95%CI： 0.571-0.859， P < 0.001）. Subgroup analysis yielded the following results： the OS-1 subgroup presented a reduced Day 3 usable embryo rate （P = 0.003）； the OS-3 and OS-4 subgroups showed decreased high-quality blastocyst rates （P = 0.004， P = 0.037 respectively）； the OS-5 subgroup had a lower Day 3 good-quality embryo rate （P = 0.045）. No statistically significant differences were detected between groups in the OS-2 and OS-6 subgroups（P > 0.05）. Conclusions Single-step culture media generally yield comparable early embryonic development outcomes to sequential culture media. However， certain single-step formulations exhibit slightly lower rates of high-quality blastocyst formation alongside higher rates of Day 3 embryonic cell fusion， indicating that individualized medium selection is required in clinical practice.

Key words: in vitro fertilization, embryo culture medium, sequential culture, single-step culture, high-quality blastocyst rate, blastomere fusion