Effect of the Timing of Frozen Embryo Transfer on Pregnancy Outcomes in High Ovarian Response Patients Undergoing Freeze-All Strategy

Abstract

Background: Frozen embryo transfer (FET) can greatly improve the pregnancy outcomes for high ovarian response (HOR) population. However, it is not known whether the impaired endometrial receptivity derived from controlled ovarian hyperstimulation (COH) can be fully recovered in the first menstrual cycle after oocyte retrieval, and whether the timing of FET is a risk factor on pregnancy outcomes in HOR population undergoing freeze-all strategy.

Methods: A retrospective cohort study to compare the pregnancy outcomes of the immediate and delayed FET groups in HOR population undergoing freeze-all strategy. Propensity score matching was used to make the potential risk factors of the immediate and delayed FET groups comparable. Multivariable regression analysis was used to study the effect of the timing of FET on pregnancy outcomes in the entire cohort and propensity score-matched cohort, even in different COH protocol cohorts as subgroup analysis.

Results: We showed that the immediate FET group were no worse than delayed FET group in the entire cohort [clinical pregnancy rate (CPR), adjusted odd ratio (OR), 0.942, 95% confidence interval (CI), 0.784-1.133; spontaneous abortion rate (SAR), adjusted OR, 1.118, 95% Cl (0.771-1.623); live birth rate (LBR), adjusted OR, 1.060, 95% Cl (0.886-1.267)]. The same results were obtained by χ² test in the propensity score-matched cohort (CPR, 60.5% versus 63.5%; SAR, 11.6% versus 12.3%; LBR, 48% versus 49.3%) (P > 0.05). Subgroup analysis indicated that pregnancy outcomes of immediate FET were non-inferior to delayed FET in short-acting gonadotropin-releasing hormone agonist (GnRH-a) long protocol (P > 0.05). The SAR of the immediate FET group were lower than that of the delayed FET group in GnRH antagonist protocol (adjusted OR, 0.646, 95% CI, 0.432-0.966) and long-acting GnRH-a long protocol (adjusted OR, 0.375, 95% CI, 0.142-0.990) (P < 0.05), no differences were observed in CPR and LBR (P > 0.05).

Conclusions: These findings indicate that immediate FET might not affect pregnancy outcomes in HOR patients undergoing freeze-all strategy. Delaying FET could increase the SAR in GnRH-ant and long-acting GnRH-a long protocols.

Background

Controlled ovarian hyperstimulation (COH) is the key step in in vitro fertilization and embryo transfer (IVF-ET). High ovarian response (HOR) refers to the abnormal sensitivity of the ovary to gonadotropin, which leads to simultaneous development of multiple follicles and increases the risk of ovarian hyperstimulation syndrome (OHSS) [1]. Supraphysiological steroid hormones during COH affect endometrial receptivity by changing the endometrial immune environment and gene expression [2–4], resulting in poor pregnancy outcomes [5]. Adopting the freeze-all strategy in HOR patients can greatly reduce the risk of OHSS and avoid the influence of COH on endometrial receptivity [6]. An increasing number of studies have confirmed that frozen embryo transfer (FET) has better pregnancy and perinatal outcomes than fresh embryo transfer [7–9].

However, the best time to perform FET following COH in HOR patients is controversial in clinical work. Postponement of FET would increase the anxiety of patients [10]; in the immediate FET cycle, poor endometrial receptivity or physical condition may not be fully recovered to the pre-stimulation state, which may affect pregnancy outcomes [11]. It is not clear whether the detrimental effects on endometrial receptivity caused by COH could sustain over a long period of time, up to the subsequent menstrual cycle, especially in patients with HOR who are most affected by COH. Moreover, the use of different gonadotropin-releasing hormone (GnRH) analogues in the process of COH disturb the reproductive endocrine physiology by inhibiting pituitary function in different ways and degrees [12], and it is controversial whether the timing of FET affects pregnancy outcomes in different COH protocols [11, 13].

Thus, this study aimed to investigate whether the FET timing affect the pregnancy outcomes in patients with HOR undergoing freeze-all strategy, and whether different COH protocols affect pregnancy outcomes in the subsequent FET cycle, and to provide reference results for the HOR population to choose the optimal time to start FET.

Materials And Methods

Results

Potential risk factors between immediate and delayed FET groups in the entire and propensity score-matched cohort

A total of 2128 HOR patients adopting freeze-all strategy underwent their first IVF/ICSI cycle (Fig. 1). There are 1130 patients in the immediate FET group and 998 patients in the delayed FET group. Patients’ and IVF characteristics in the immediate and delayed FET groups, which as potential risk factors, were presented in Table 1. Before matching, the distribution of these risk factors were not absolutely balanced. The distribution of COH protocol, number of embryo transferred, embryo stage and multiple pregnancies were significantly different between the two groups (P < 0.05). No significant differences were found in maternal age, body mass index (BMI), insemination method, and infertility causes (P > 0.05). We obtained 1366 patients by propensity score matching, and all potential risk factors and the pregnancy outcome of multiple pregnancies among them are balanced and comparable (Table 1).

Abbreviations

BMI, body mass index; COH, controlled ovarian hyperstimulation; FET, frozen embryo transfer; GnRH-a, gonadotrophin-releasing hormone agonist; GnRH-ant, gonadotrophin-releasing hormone antagonist; HRT, hormone replacement therapy; IVF, in vitro fertilization; ICSI, intracytoplasmic sperm injection. Data are presented as number (%).

^aImmediate FET and ^bDelayed FET means FET took place either within the first menstrual cycle following oocyte retrieval or afterwards.

*The predictors of propensity score matching were maternal age, BMI, insemination method, COH protocol, number of embryo transferred, embryo stage and cause of infertility. The match tolerance was set to 0.000001. Categorical variables were compared with χ² test.

^# P value of differences of different trigger drugs in GnRH-ant protocol.

Multivariable logistic regression analysis on FET timing and potential risk factors for pregnancy outcomes in the entire before-matching cohort

Multivariable logistic regression analysis on the entire before-matching cohort demonstrated no statistically differences on pregnancy outcomes between the immediate and delayed FET groups [CPR, adjusted odd ratio (OR), 0.942, 95% confidence interval (CI), 0.784–1.133; SAR, adjusted OR, 1.118, 95% Cl (0.771–1.623); LBR, adjusted OR, 1.060, 95% Cl (0.886–1.267)] (P > 0.05), which were adjusted for maternal age, BMI, COH protocol, insemination method, number of embryo transferred, embryo stage, trigger type, and cause of infertility (Table 2).

Abbreviations BMI, body mass index; FET, frozen embryo transfer; GnRH-a, gonadotrophin-releasing hormone agonist; GnRH-ant, gonadotrophin-releasing hormone antagonist; IVF, in vitro fertilization; ICSI, intracytoplasmic sperm injection; OR, odds ratio; CI, confidence interval; NS, not significant.

^aImmediate FET and ^bdelayed FET means FET took place either within the first menstrual cycle following oocyte retrieval or afterwards.

*Using the multivariable logistic regression and adjusting for maternal age, BMI, insemination method, COH protocol, trigger type, number of embryo transferred, embryo stage and cause of infertility.

Discussion

In clinical work, when to start the FET cycle after COH is controversial, especially for patients with HOR, who are most affected by COH. As known, in the COH process, increased levels of supraphysiological steroid hormones and premature progesterone affected the gene expression and immune environment of the endometrium, which altered the embryo-endometrium asynchrony and negatively affected endometrial receptivity, reducing the CPR and LBR [5, 19]. Moreover, the secretory activity factors produced by the residual luteal cysts derived from COH would last longer after oocyte retrieval in HOR patients. However, no study has investigated the specific duration of these adverse effects. To avoid this concern, some clinicians recommended the conservative scheme to start the FET cycle at the second or third withdrawal bleeding after oocyte retrieval, which would undoubtedly prolong the IVF treatment period and increase the anxiety of patients who had experienced infertility for many years, increasing their mental and economic loses and affecting their pregnancy outcomes [10, 20]. In our retrospective study, we only selected the HOR population who adopted HRT as endometrial preparation protocol in the past 5 years, and showed no significant differences on CPR, SAR, and LBR between the immediate and delayed FET groups. We believe that it is not accurate to assume that COH will still affect endometrial receptivity in the first withdrawal bleeding cycle after oocyte retrieval and that the HOR population do not have to wait for several menstrual cycles to FET after freeze-all strategy.

In the process of COH, different GnRH analogues have different degrees and properties of inhibition effects on hypothalamic-pituitary-ovarian axis, it also has different effects on corpus luteum [21], which might have impact on endometrial receptivity and pregnancy outcomes [22]. A retrospective study have showed that immediate FET had similar CPR to delayed FET in patients with GnRH-ant protocol [13], which is in agreement with our subgroup results. However, our results are contrary to a population-based study on short-acting GnRH-a long protocol, which found that delayed FET was better for pregnancy outcomes, they believed that the initial flare up effect of short acting GnRH-a during the down regulation period caused an early rise of progesterone, which affected the outcomes in immediate FET cycle [11]. An important limitation of that study is the small sample size, 67 patients in immediate FET group and 62 in delayed FET group. In this study, 1000 patients with short acting GnRH-a long protocol were studied (434 in immediate FET group and 566 in delayed FET group), and we found that the timing of FET did not affect pregnancy outcomes in short-acting GnRH-a long protocol. However, in the GnRH-ant and long-acting GnRH-a long protocol, we found that the SAR in delayed FET group was significantly higher than that in immediate FET group. Among all the COH protocols, the GnRH-ant protocol has the shortest treatment period, which oocyte retrieval take place after 8–10 days of ovarian stimulation by mean; the short-acting GnRH-a long protocol needs 14 days of down regulation on that basis; while the long-acting GnRH-a long protocol needs a down regulation for more than 20 days. The immediate FET cycle in GnRH-ant protocol can obtain the shortest treatment period, while the delayed FET cycle in long-acting GnRH-a long protocol has the longest treatment period. We know that psychological factors are important factors leading to infertility and spontaneous abortion, its potential impact on neuroendocrine and immune changes could affect early pregnancy risk [23, 24]. We consider that the longer the treatment period, the more anxious the patients are, which could be the cause of the increase in the SAR. Moreover, long acting GnRH-a can effectively improve endometrial receptivity [22], which may be the other reason for the lower SAR in the immediate FET group in long acting GnRH-a long protocol. It is worth mentioning that one research have shown that residual luteal cysts may increase the expression of relaxin in circulation [25], which is related to the endometrial angiogenesis and prevent recurrent abortion [26]. Therefore, the effects of residual luteal cysts in immediate FET cycles that we had previously worried about may be beneficial to endometrial receptivity and pregnancy outcomes.

The limitation of this study lies in the retrospective nature as well as the possibility of unmeasured confounding factors such as smoking habits and alcohol consumption. Although we have obtained many cases with HOR and made the immediate and delayed FET groups comparable by propensity score matching, we have lost 762 cases without successful matching in this process. However, we do not know whether these cases will affect the actual situation. Notably, the pregnancy outcomes we have studied were not the only endpoint, other obstetric outcomes and neonatal outcomes should also be noticed.

In summary, this study implied that immediate FET may not affect the pregnancy outcomes in HOR population undergoing freeze-all strategy although their pelvic environment were not fully recovered. Intended to delay FET in HOR population may increase anxiety of infertility couples and increase the SAR. Clinicians and patients with HOR do not have to worry about FET on the immediate cycle, and they can arrange the start time of FET cycle on their own convenience and desire, making the IVF treatment process more relax and efficient. However, more studies are needed to further investigate the effect of FET timing on other obstetric and neonatal outcomes in the HOR population.

Conclusion

Immediate FET may not have adverse effect on pregnancy outcomes in HOR population undergoing freeze-all strategy. Delaying FET could increase the SAR in GnRH-ant and long-acting GnRH-a long protocols. Intentionally delayed FET by clinicians or HOR patients is not beneficial to pregnancy outcomes.

List Of Abbreviations

BMI: Body mass index; COH: Controlled ovarian hyperstimulation; CI: Confidence interval; CPR: Clinical pregnancy rate; FET: Frozen embryo transfer; GnRH-a: Gonadotropin-releasing hormone agonist; GnRH-ant: Gonadotropin-releasing hormone antagonist; HCG: human chorionic gonadotropin; HOR: High ovarian response; HRT: Hormone replacement therapy; ICSI: Intracytoplasmic sperm injection; IVF-ET: In vitro fertilization and embryo transfer; LBR: Live birth rate; OR: Odd ratio; OHSS: Ovarian hyperstimulation syndrome; SAR: Spontaneous abortion rate.

Declarations

Ethics approval and consent to participate

All the data used in this study were retrospective clinical data, which were approved by the Institutional Review Board at China Medical University (reference number 2020PS006F).

Consent for publication

Not applicable.

Availability of data and materials

The datasets generated and analysed during the current study were used under license for the current study, and so are not publicly available.

Competing interests

The authors declare that they have no competing interests.

Funding

This work was supported by the National Natural Science Foundation of China (grant number 82071607 and 81671423), the National Key Research and Development Program of China (grant number 2018YFC1003100), Liaoning Revitalization Talents Program (grant number XLYC1907071), Liaoning Provincial Key Research and Development Program (grant number 2018225090), the Fok Ying Tung Education Foundation (grant number 151039).

Authors^’ contributions

D.L. and X.X.W. conceived and designed the study. N.Z., N.N.Z., Y.Z.G. performed the data collection and analysis. D.L., X.X.W., N.Z. and Y.Z.G. wrote the paper. All authors read and approved the final manuscript.

Acknowledgments:

Not applicable.

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