Objective:To analyze the clinical outcomes of the first frozen-thawed embryo transfer (FET) in patients <35 years old applying natural cycle (NC) and hormone replacement therapy (HRT).Methods:A retrospective cohort study was conducted to analyze 4 814 young infertility patients who underwent the first FET in Reproductive Medicine Center of the First Affiliated Hospital of Zhengzhou University from January 2016 to June 2021. According to different endometrial preparation protocols, they were divided into 2 groups: NC group and HRT group, who were matched the baseline data using 1∶1 propensity score matching (PSM). After the matching, the two groups of their baseline data, pregnancy outcomes and perinatal outcomes were compared, and then we adjusted the confounding factors which affect live birth rate by univariate and multivariate logistic regression analysis. Based on antral follicle count (AFC), number of embryos transferred and number of high-quality embryos transferred, the effect of NC and HRT on the live birth rate were further analyzed.Results:Before PSM, 2 131 patients in NC group and 2 683 patients in HRT group were included. The differences in female age, male age, body mass index (BMI), basal follicle-stimulating hormone (bFSH), anti-Müllerian hormone (AMH), AFC, endometrial thickness on conversion day, and number of embryos transferred were all statistically significant between the two groups (all P<0.05). And the differences in number of high-quality embryos transferred and type of embryos transferred between the two groups were not statistically significant (all P>0.05). After PSM, 1 441 patients in each of NC group and HRT group were included, and there were no significant differences in their baseline characteristics such as female age, male age and BMI between the two groups (all P<0.05). The live birth rate [50.66% (730/1 441)] and the clinical pregnancy rate [60.31% (869/1 441)] in NC group were significantly higher than those in HRT group [44.69% (644/1 441), P=0.001; 54.27% (782/1 441), P=0.001], and the incidence of very low birth weight in NC group was significantly lower than that in HRT group, and there were no statistical significances in other indicators between the two groups (all P>0.05). After adjusting confounders including bFSH, AMH, AFC, endometrial thickness on conversion day, number of embryos transferred and high-quality embryos transferred using multivariate logistic regression analysis, the results showed that NC was an independent protective factor for live birth rate in the first FET cycle ( aOR=1.280, 95% CI: 1.103-1.486, P=0.001). Stratified analysis showed that those with AFC<11, AFC 11-12, 2 embryos transferred and 2 high-quality embryos tranferred in NC group had significantly higher live birth rate [49.03% (151/308), 49.09% (349/711), 56.38% (442/784), 57.85% (350/605)] than those in HRT group [36.36% (120/330), P=0.001; 43.14% (286/663), P=0.027; 48.97% (379/774), P=0.003; 48.68% (294/604), P=0.001]. Conclusion:NC-FET had higher live birth rate and clinical pregnancy rate than HRT-FET in young patients.

Objective:To analyze the clinical outcomes of the first frozen-thawed embryo transfer (FET) in patients <35 years old applying natural cycle (NC) and hormone replacement therapy (HRT).Methods:A retrospective cohort study was conducted to analyze 4 814 young infertility patients who underwent the first FET in Reproductive Medicine Center of the First Affiliated Hospital of Zhengzhou University from January 2016 to June 2021. According to different endometrial preparation protocols, they were divided into 2 groups: NC group and HRT group, who were matched the baseline data using 1∶1 propensity score matching (PSM). After the matching, the two groups of their baseline data, pregnancy outcomes and perinatal outcomes were compared, and then we adjusted the confounding factors which affect live birth rate by univariate and multivariate logistic regression analysis. Based on antral follicle count (AFC), number of embryos transferred and number of high-quality embryos transferred, the effect of NC and HRT on the live birth rate were further analyzed.Results:Before PSM, 2 131 patients in NC group and 2 683 patients in HRT group were included. The differences in female age, male age, body mass index (BMI), basal follicle-stimulating hormone (bFSH), anti-Müllerian hormone (AMH), AFC, endometrial thickness on conversion day, and number of embryos transferred were all statistically significant between the two groups (all P<0.05). And the differences in number of high-quality embryos transferred and type of embryos transferred between the two groups were not statistically significant (all P>0.05). After PSM, 1 441 patients in each of NC group and HRT group were included, and there were no significant differences in their baseline characteristics such as female age, male age and BMI between the two groups (all P<0.05). The live birth rate [50.66% (730/1 441)] and the clinical pregnancy rate [60.31% (869/1 441)] in NC group were significantly higher than those in HRT group [44.69% (644/1 441), P=0.001; 54.27% (782/1 441), P=0.001], and the incidence of very low birth weight in NC group was significantly lower than that in HRT group, and there were no statistical significances in other indicators between the two groups (all P>0.05). After adjusting confounders including bFSH, AMH, AFC, endometrial thickness on conversion day, number of embryos transferred and high-quality embryos transferred using multivariate logistic regression analysis, the results showed that NC was an independent protective factor for live birth rate in the first FET cycle ( aOR=1.280, 95% CI: 1.103-1.486, P=0.001). Stratified analysis showed that those with AFC<11, AFC 11-12, 2 embryos transferred and 2 high-quality embryos tranferred in NC group had significantly higher live birth rate [49.03% (151/308), 49.09% (349/711), 56.38% (442/784), 57.85% (350/605)] than those in HRT group [36.36% (120/330), P=0.001; 43.14% (286/663), P=0.027; 48.97% (379/774), P=0.003; 48.68% (294/604), P=0.001]. Conclusion:NC-FET had higher live birth rate and clinical pregnancy rate than HRT-FET in young patients.