Beyond morphology: re-expansion dynamics and transfer timing influence twin pregnancy rates in euploid blastocyst transfers

The factors influencing implantation and twin pregnancy outcomes following dual euploid blastocyst transfers (DET) are not fully understood. This prospective cohort study analyzed maternal and embryological factors associated with implantation success in DET cycles using chromosomally normal blastocysts. All embryos underwent preimplantation genetic testing for aneuploidy (PGT-A) to confirm chromosomal normality. We investigated relationships between maternal age, blastocyst morphological grade, day of trophectoderm biopsy, post-thaw re-expansion, and thaw-to-transfer intervals with implantation success. Greater post-thaw re-expansion (≥ 100%) was more frequent in twin gestations (OR = 1.241, p = 0.033), and shorter thaw-to-transfer intervals were associated with more twin implantations (OR = 0.99, p = 0.006). No significant association was found between maternal age, morphological grading and day of biopsy with twin pregnancy rates. Improved blastocyst re-expansion and shorter thaw-to-transfer intervals are associated with successful twin implantation, highlighting the importance of optimizing pre-transfer conditions beyond chromosomal and morphological assessment.