Physician-Selected Timing of Frozen Blastocyst Transfer and Clinical Pregnancy Outcomes: a Retrospective Cohort Study

Abstract

Background: Programmed frozen blastocyst transfer requires coordination between progesterone-driven endometrial maturation and embryo development. More than one progesterone exposure window is used in routine practice, and the treating physician may select timing for an individual cycle. This study evaluated whether physician-selected timing was associated with ultrasound-confirmed clinical pregnancy.
Methods: This retrospective single-center cohort included 160 frozen embryo transfer cycles performed at the Georgian-German Reproduction Center during May 2026. Inclusion criteria were endometrial thickness at least 8 mm, serum progesterone below 1 ng/mL before progesterone initiation, and vaginal micronized progesterone 600 mg daily. Cycles using endometrial receptivity testing were excluded. The primary analysis excluded four Day-3 transfers performed before 120 hours, four mixed Day-5 and Day-6 transfers, and two blastocyst transfers performed after 136 hours, leaving 150 Day-5 or Day-6 transfers at 120-126 or 133-136 hours. Clinical pregnancy was defined as ultrasonographic visualization of at least one gestational sac. Fisher exact tests and multivariable logistic regression were used.
Results: Clinical pregnancy occurred in 42 of 80 transfers at 120-126 hours (52.5%) and 34 of 70 transfers at 133-136 hours (48.6%). The later window was not significantly associated with clinical pregnancy in unadjusted analysis (odds ratio 0.85, 95% confidence interval 0.45-1.62; P=0.744) or after adjustment for embryo day, recipient type, oocyte source, oocyte age, and body mass index (adjusted odds ratio 0.84, 95% confidence interval 0.43-1.63; P=0.606). No significant interaction was identified by recipient type, body mass index, or embryo day.
Conclusion: Within the two evaluated exposure windows and standardized programmed-cycle  protocol, physician-selected timing was not significantly associated with clinical pregnancy. The findings support continued physician-directed selection of these timing options for appropriately selected cycles, but do not establish equivalence or superiority.