Ongoing pregnancy rates (OPRs) after warmed blastocyst transfer (WBT) in a true-natural cycle (t-NC) are similar using six different luteinizing hormone (LH) surge criteria

Abstract Study question Does timing of WBT in t-NC differ according to six different commonly definitions for LH surge, and if so, do differences in timing impact OPRs? Summary answer Performing WBT on follicular collapse+5 days is equivalent to LH surge +7/+8 /+9 days in terms of OPRs, using six different definitions of LH surge. What is known already Pinpointing the day of ovulation, usually by documentation of the LH surge, and less commonly by transvaginal-ultrasonography is crucial for timing WBT in t-NC to maximize reproductive success. However, there is no consensus on the definition of the LH surge, and the most commonly used six LH-surge definitions are LH ≥ 10, ≥15, ≥17, ≥20 IU/L, ≥1.8-fold, and ≥2-fold increase from baseline. The usual practice is to schedule warmed blastocyst transfer on LH-surge +6 days. Study design, size, duration Prospective monitoring of 115 WBT cycles performed during January 2017-October 2021. The goals of the study were i)to assess how frequently and to what extent there would be a change in WBT related to the day of the LH surge, using the six different definitions of LH surge, compared to follicular collapse +5 days; ii)for each definition of the LH surge to compare OPRs of different WBT timings related to the day of LH surge. Participants/materials, setting, methods Staying locally and having regular menstrual cycles were the main criteria to perform t-NC. For t-NC, serial serum endocrine (LH, estradiol, and progesterone) and transvaginal ultrasonographic monitoring started on cycle days 8-10. Following precise documentation of follicular collapse by ultrasound, WBT was performed on follicular collapse +5 days. All included cycles were t-NC without human chorionic gonadotropin trigger or luteal phase support administration. Main results and the role of chance A total of 115 t-NC cycles were included for the first part of the study, determining the impact of different definitions of the LH-surge for the day of WBT. Our reference timing of follicular collapse +5 days would be equivalent to LH-surge +6 days in only 5.2%-41.2% of the cycles employing the six different LH-surge definitions. In contrast, the reference timing was comparable to LH surge +7 days in the majority of cycles (46.1%-70.8%) and less commonly to LH-surge +8 days (1.8%-38.3%) and +9 days (0%-10.4%). For the second part of the study, a total of 94 cycles were analyzed; 15 cycles were excluded as these cycles constituted 2nd or 3rd t-NC cycles; four cycles due to low serum progesterone (<7 ng/ml) on WBT-1 day and two cycles due to failure of survival after warming. For each LH-surge definition, OPRs were comparable among the different WBT timings related to the LH-surge (+6/+7/+8/+9 days). When logistic regression analysis was performed, taking LH-surge + 6 days as the reference, a change in timing was not an independent predictor of OPR for all six different definitions of the LH-surge. Limitations, reasons for caution Assignment of WBT timings related to LH-surge by our standard policy (follicular collapse +5 days), rather than by randomization, is a limitation. Other limitations include single daily measurements of serum LH and limited sample size. Wider implications of the findings Differences in warmed blastocyst timing related to the LH surge (LH surge +6/+7/+8/+9) are associated with comparable reproductive outcomes in t-NC, reflecting the flexibility of the window of implantation. Further, trials are warranted to delineate the best tool and timing of FET for warmed blastocyst transfer in t-NC. Trial registration number Not applicable