{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,6,2]],"date-time":"2026-06-02T09:46:49Z","timestamp":1780393609683,"version":"3.54.1"},"reference-count":51,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2025,5,14]],"date-time":"2025-05-14T00:00:00Z","timestamp":1747180800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Endocrinol."],"abstract":"<jats:sec><jats:title>Background<\/jats:title><jats:p>To compare the clinical outcomes between frozen-thawed cleavage embryo transfer and frozen-thawed day 6 blastocyst transfer in patients younger than 35 using the freeze-all strategy without day 5 blastocyst formation.<\/jats:p><\/jats:sec><jats:sec><jats:title>Methods<\/jats:title><jats:p>This was a retrospective observational analysis performed between January 2018 and December 2022 at the Reproductive and Genetic Specialist Hospital of the First Affiliated Hospital of Zhengzhou University. A total of 576 patients younger than 35 who used the \u201cfreeze-all strategy\u201d but produced no day 5 blastocysts were recruited. The patients were divided into 3 groups according to the number and stage of the transferred embryos: double cleavage-stage embryos (Group A), single good-quality day 6 blastocysts (Group B) and single inferior-quality day 6 blastocysts (Group C),and several pregnancy outcomes were measured.<\/jats:p><\/jats:sec><jats:sec><jats:title>Results<\/jats:title><jats:p>Groups A and B exhibited significantly higher chemical (73.7%, 67.0% versus 51.9%) and clinical pregnancy rates (69.0%, 59.4% versus 44.2%) than Group C. The implantation rate was significantly higher in Group B than in Groups A and C (59.4% versus 45.7%, 43.5%). The live birth rate was significantly higher in Group A than in Group C (59.2% versus 48.1%). The multiple pregnancy rate was significantly higher in Group A than in Groups B and C (34.4% versus 1.6%, 1.5%). The early miscarriage rate was significantly higher in Group C than in Group A and Group B (23.5% versus 8.7%, 12.7%). Premature delivery rates, late miscarriage rates and ectopic pregnancy rates were comparable across groups.<\/jats:p><\/jats:sec><jats:sec><jats:title>Conclusions<\/jats:title><jats:p>A single good quality day 6 blastocyst transfer was the preferable strategy for the freeze-all strategy patients who younger than 35 and without day 5 blastocyst formation.<\/jats:p><\/jats:sec>","DOI":"10.3389\/fendo.2025.1473854","type":"journal-article","created":{"date-parts":[[2025,5,14]],"date-time":"2025-05-14T04:13:11Z","timestamp":1747195991000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":1,"title":["Frozen-thawed double cleavage-stage or frozen-thawed single day 6 blastocyst stage embryo transfer: which is preferable for patients younger than 35 without day 5 blastocyst 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