ArticleJournal of assisted reproduction and genetics2026
Not all Day-4 embryos are equal: developmental stage predicts outcomes after single embryo transfer.
Article in Journal of assisted reproduction and genetics, 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
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6 authors.
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Abstract
purposeEvaluate outcomes of Day-4 embryo transfer according to embryonic developmental stage.
methodsThis retrospective single-center study included fresh IVF/ICSI cycles with day-4 single embryo transfer between January 2021 and July 2025. Cycles with double embryo transfer, rescue ICSI, or missing follow-up were excluded. Transfers were divided into three groups based on embryonic stage at transfer: cleavage, morula or blastocyst-stage. Demographic and cycle characteristics, as well as clinical outcomes, were compared. Clinical pregnancy was the primary outcome.
resultsA total of 669 day-4 single fresh embryo transfer cycles were included: 130 cleavage-stage (19.4%), 449 morula-stage (67.1%), and 90 blastocyst-stage (13.5%). Compared to the morula stage, the cleavage-stage group was characterized by an older age, lower oocyte yield and fewer 2PN embryos, while the blastocyst-stage group was characterized by a younger age and a similar oocyte yield and 2PN embryos. Clinical pregnancy rates increased significantly with advancing embryonic stage (10.0%, 32.96%, and 48.89%, respectively; p < 0.05). In multivariable analysis, only increasing maternal age (OR 0.92, 95% CI 0.88-0.96) and cleavage-stage transfer (OR 0.37, 95% CI 0.18-0.78) remained negatively associated with clinical pregnancy.
conclusionsFresh day 4 embryo transfer outcomes are strongly influenced by embryonic developmental stage. Cleavage-stage embryos on day 4 are associated with poor pregnancy rates and may benefit from extended culture. In contrast, day 4 morula or blastocyst transfer represents a viable clinical option, particularly when logistical constraints preclude day 5 transfer.
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