Comparison of the efficacy of letrozole-ovulation induction cycle, natural cycle, and hormonal replacement therapy cycle in cryopreserved embryo transfer: a retrospective cohort study.

Objective: To compare the clinical efficacy of letrozole-induced ovulation cycles, natural cycles, and Hormonal Replacement Therapy cycle in frozen embryo transfer (FET), and to investigate the effects of different endometrial preparation protocols on pregnancy outcomes, hormonal levels, and endometrial receptivity. Methods: This study was a single-center retrospective cohort study that enrolled a total of 3,068 FET patients who visited our center between January 1, 2023, and December 30,2025. Patients were divided into three groups based on the endometrial preparation protocol: Group A (letrozole ovulation cycle, n=417), Group B (natural cycle, n=887), and Group C (Hormonal Replacement Therapy cycle, n=1,764). The primary outcome measures were clinical pregnancy rate and live birth rate. Secondary outcome measures included early miscarriage rate, ectopic pregnancy rate, serum estradiol and progesterone levels, endometrial thickness, endometrial morphology, and the incidence of endometrial compression. The impact of different protocols on pregnancy outcomes was evaluated using univariate analysis and multivariate logistic regression analysis. Results: There were no statistically significant differences in baseline characteristics (age, BMI, type of infertility, AMH, etc.) among the three groups (P>0.05). The clinical pregnancy rate (52.5%) and live birth rate (45.3%) in Group A were higher than those in Group C (48.6% and 40.9%, respectively), but the differences were not statistically significant (P>0.05), and were comparable to those in Group B (50.8% and 43.2%, respectively; P>0.05). The early miscarriage rates in Groups A and B (10.5% and 10.4%, respectively) were lower than that in Group C (11.2%), but the difference was not statistically significant (P>0.05). In terms of hormone levels, the E2 levels on the endometrial transformation day were significantly lower in Groups A and B compared to Group C (P<0.001), while the P levels were significantly higher (P<0.001). The proportion of type A endometrium in Groups A and B (75.5% and 77.1%, respectively) was significantly higher than that in Group C (72.1%; P<0.05). The incidence of endometrial cysts (EC) in Groups A and B (35.0% and 34.0%, respectively) was significantly higher than that in Group C (28.6%; P<0.001). Multivariate logistic regression analysis revealed that, compared with Group C, Groups A (aOR=1.254,95% CI 1.032-1.525, P = 0.023) and B (aOR=1.312,95% CI 1.045-1.648, P = 0.019) were independent positive predictors of clinical pregnancy. Conclusion: Both letrozole ovulation induction cycles and natural cycles achieved similar clinical pregnancy rates and live birth rates in FET, with both values being numerically superior to those observed in hormone replacement therapy cycles, but non-significant. These two protocols may improve pregnancy outcomes by providing a hormonal environment closer to physiological conditions and enhanced endometrial receptivity. For patients with ovulatory function, letrozole ovulation induction cycles and natural cycles are the preferred endometrial preparation regimens for FET.