[Impact of progesterone concentration on hCG trigger day on clinical outcomes with cleavage-stage embryo transfer in in vitro fertilization cycles with an antagonist protocol].

<b>Objective:</b> To investigate the impact of the progesterone concentration on human chorionic gonadotropin (hCG) trigger day in fresh cycles versus thawed transfer cycles (the freeze-all strategy) with an antagonist protocol, and to compare the differences in clinical outcomes. <b>Methods:</b> This retrospective cohort study included a total of 2 165 cycles conducted at Henan Provincial People's Hospital with cleavage-stage embryo (at least one top-quality) transfer between January 2017 and December 2023, with serum progesterone levels on hCG trigger day all≤6.34 nmol/L (i.e. 2 ng/ml). Multivariate logsitic regression analysis and curve fitting were performed based on different serum progesterone levels on hCG trigger day [≤3.17 nmol/L (i.e. 1 ng/ml) or 1-2 ng/ml]. <b>Results:</b> Multivariate regression analysis, by using cycle type (either fresh or frozen-thawed cycle) as the exposure variable, showed that the clinical pregnancy rate (≤1 ng/ml: <i>OR</i>=0.93, 95%<i>CI</i>: 0.75-1.14; 1-2 ng/ml: <i>OR</i>=1.05, 95%<i>CI</i>: 0.58-1.87) and live birth rate (≤1 ng/ml: <i>OR</i>=0.90, 95%<i>CI</i>: 0.71-1.13; 1-2 ng/ml: <i>OR</i>=1.53, 95%<i>CI</i>: 0.79-3.00) had no statistically significant differences in group of progesterone concentration ≤1 ng/ml or in group of 1-2 ng/ml. Using serum progesterone levels on hCG trigger day as a continuous variable for curve fitting analysis, the clinical pregnancy rate in fresh or thawed cycles showed no significant changes with increasing progesterone levels. <b>Conclusions:</b> In the antagonist protocol with cleavage-stage embryo transfer (at least one top-quality), when the serum progesterone level on hCG day is ≤2 ng/ml, there are no significant differences in clinical outcomes between thawed cycles and fresh cycles, including clinical pregnancy rate and live birth rate. Transferred in fresh cycles or choosing the freeze-all strategy could be selected based on the actual situation of the patients.