Natural cycles achieve better pregnancy outcomes than artificial cycles in non-PCOS women undergoing vitrified single-blastocyst transfer: a retrospective cohort study of 6840 cycles.

Purpose To identify the optimal method for endometrial preparation in vitrified single-blastocyst transfer (VSBT) cycles.

This was a retrospective cohort study for non-PCOS patients who underwent VSBT cycles from March 2015 to November 2019 in an academic reproductive medical center. A total of 6840 VSBT cycles were enrolled and classified into two groups according to different endometrial preparation methods.

The non-PCOS patients who underwent VSBT showed a significantly higher clinical pregnancy rate (61.96% vs 56.85%, p < 0.001) and live birth rate (49.09% vs 39.86%, p < 0.001), as well as a statistically lower early miscarriage rate (12.02% vs 18.08%, p < 0.001) in the natural cycle (NC) group compared with the artificial cycle (AC) group. Multivariable analysis further confirmed that NC was associated with an increased likelihood of clinical pregnancy (adjusted odds ratio (aOR) 0.852, 95% confidence interval (CI) 0.765–0.949, p = 0.004) and live birth (aOR 0.746, 95% CI 0.669–0.832, p < 0 .001), but decreased early miscarriage occurrence (aOR 1.447, 95% CI 1.215–1.724, p < 0.001) compared to AC.

Our study demonstrated that non-PCOS patients could benefit from NC in vitrified blastocyst transfer. Increased clinical pregnancy rate and decreased early miscarriage rate led to a significantly higher live birth rate in NC patients compared with AC with our present protocol. Supplementary Information The online version contains supplementary material available at 10.1007/s10815-022-02424-0.

Vitrified single-blastocyst transfer, Frozen embryo transfer, Endometrial preparation, Live birth, Pregnancy outcome

It has been almost 40 years since the first successful freeze–thaw of human embryos and the first live birth after frozen embryo transfer (FET) were reported in the 1980s [1]. Improvements in laboratory techniques such as vitrification and blastocyst culture have led to a rapid increase in the number of FET cycles. At present, FET has become common practice in assisted reproduction technology (ART) owing to its remarkable advantages such as reducing the risk of ovarian hyperstimulation syndrome (OHSS), preserving fertility, increasing cumulative live birth rate, avoiding exposure of the endometrium to the adverse sequelae of ovarian stimulation, and offering the opportunity to detect genetic anomalies [2–4]. Furthermore, the adoption of the concept of single embryo transfer to reduce multiple pregnancy rates and associated morbidities has also encouraged the application of single-blastocyst transfer, which could maintain high live birth rates while reducing the risk of multiple pregnancy [5]. Currently, we use vitrified single-blastocyst transfer (VSBT) in most frozen embryo transfer cycles in our center. To achieve an optimal endometrial environment and successful embryo transfer, endometrial receptivity should be well scheduled to match with the stage of embryo development [6]. There are two classical and commonly used methods. An artificial cycle (AC) is the most common protocol for endometrial preparation [7, 8], in which, a proper endometrial environment is created by programmed administration of exogenous estradiol and progesterone. In a natural cycle (NC), embryos are transferred after ovulation with or without induction by human chorionic gonadotropin (hCG). NC gains preference for its low-cost and reduced use of medicines in the clinic, while AC is favored for its lower cancellation rate and greater flexible for scheduling [9, 10]. Recently, many reports have highlighted that NC has an obvious advantage in decreasing the risk of adverse obstetric outcomes [11–16]. However, controversy over which protocol achieves superior pregnancy outcomes continues [17–19], partly owing to the diversity of clinical patients, the difference in methods used for embryo cryopreservation, and the selection of developmental stage for cryopreservation. Decisions on clinical practice may be changed according to the development of techniques and theories. Therefore, we conducted this retrospective cohort study in non-PCOS women undergoing VSBT cycles to determine which protocol is superior for live birth and pregnancy outcomes. This study offers more evidence for the choice of endometrium preparation method during frozen embryo transfer.

Study design and population selection We set up a retrospective study on FET cycles from March 2015 to November 2019 at an academic reproductive medical center. Data of non-PCOS patients who underwent VSBT were extracted from the electronic data base of our hospital. Patients were included in our research if (i) they underwent blastocyst transfer after in vitro fertilization (IVF) with or without intracytoplasmic sperm injection (ICSI), (ii) their endometrium was prepared with NC or AC, (iii) they were aged from 20 to 37 years, and (iv) they had only one vitrified-warming blastocyst transferred. Patients with a history of ovarian surgery, cervix surgery, uterine abnormality, recurrent implantation failure (RIF), recurrent spontaneous abortion (RSA), polycystic ovary syndrome (PCOS), and endometriosis were excluded from this study. In addition, we only enrolled FET cycles of autologous embryos. Women who utilized donated sperm or oocytes were excluded. The criteria of RIF, as suggested by Chinese expert consensus on preimplantation genetic diagnosis/screening [20], are implantation failure (including biochemical pregnancy loss) after three cycles of embryo transfer, or at least 4 ~ 6 good-quality cleavage embryo transfers, or at least three good-quality blastocyst transfers. To diagnose PCOS, we used the Chinese diagnostic criteria [21, 22], which are modified Rotterdam criteria. The criteria included menstrual abnormalities (irregular uterine bleeding, oligomenorrhea, or amenorrhea) combined with either hyperandrogenism or polycystic ovaries, as validated in our Chinese population.

In NC, follicles were monitored by transvaginal ultrasonography from the 8th–12th day of the menstrual cycle. Serum or urine luteinizing hormone and serum progesterone and estrogen were tested to evaluate the exact ovulation time. Ovulation occurred either spontaneously or was induced by hCG. After ovulation was confirmed, oral progesterone (Duphastone, 10 mg, 20–40 mg/day, Solvay, the Netherlands) was administered for luteal phase support. The ovulation day was set as day 0, and the blastocyst was warmed and transferred on day 5. .In AC, 4–8 mg oral estradiol valerate (Progynova, Bayer, Germany) was administered daily for at least 10 days starting from day 2 to day 5 of the menstrual cycle. When the endometrial thickness reached ≥ 8 mm, progesterone was administered both orally (Duphastone, 10 mg, 40 mg/day, Solvay, Netherlands) and vaginally (Utrogestan, 100 mg, 200 mg/day, Besins Manufacturing, Belgium) along with the oral estradiol. The day starting progesterone was set as day 0, and blastocyst transfer was scheduled on the 5th day of progesterone administration. Luteal phase support and definition of clinical outcomes Luteal support would continue until 11–12 weeks of gestation. Clinical pregnancy was determined through transvaginal ultrasonography by detecting one or more gestational sacs. Early miscarriage was defined as the spontaneous loss of clinical pregnancy within the first 13 weeks of gestation. Live birth was defined as at least one live fetus delivered beyond 22-week gestation. Statistical analyses Continuous variables were presented as mean ± standard deviation. The differences between groups were analyzed by independent sample t-test if data was normally distributed; otherwise it was analyzed by Kruskal–Wallis test. Categorical variables were presented as frequencies and percentages, and they were analyzed by chi-square test. The NC group was taken as the reference group. Multivariate logistic regression analyses were applied to study the confounding factors related to live birth rate, clinical pregnancy rate, and early miscarriage rate. The age, body mass index (BMI), antral follicle count (AFC), primary subfertility, regular menstrual cycle, insemination method, protocol for controlled ovarian hyperstimulation (COH), number of oocytes retrieved, number of embryos frozen, protocol for FET, endometrial thickness on progesterone administration day, times of previous embryo transfer (ET), and times of previous early miscarriage were included in the analysis. All analyses were performed with SPSS software (v.25.0 for Windows; SPSS Inc., Chicago, IL, USA). The level of significance was defined as p < 0.05.

A total of 6840 VSBT cycles were included into the study and were divided into NC group (n = 4870) and AC group (n = 1970) dependent upon endometrial preparation method. A post hoc power analysis was also conducted and found the power of our study was almost 100%. The baseline characteristics of the two groups are shown in Table 1. There were significant differences between the two groups in many variables, such as body mass index (BMI), antral follicle count (AFC), regular menstruation, times of previous FET, and endometrial thickness. Table 1. | Natural cycle (n = 4870) | Artificial cycle (n = 1970) | p-value | | |---|---|---|---| | Maternal age (years) | 30.49 ± 3.64 | 30.13 ± 3.59 | < 0.001 | | BMI | 22.87 ± 3.33 | 23.74 ± 3.55 | < 0.001 | | Regular menstruation (%) | 4535(93.12) | 1487(75.48) | < 0.001 | | AFC | 13.37 ± 5.82 | 14.31 ± 6.95 | < 0.001 | | Infertility status | < 0.001 | || | Primary infertility (%) | 2274(46.69) | 914(46.40) | | | Insemination method (%) | 0.349 | || | IVF | 4140(85.01) | 1657(84.11) | | | ICSI | 730(14.99) | 313(15.89) | | | Protocol for COH | 0.002 | || | GnRH-a long protocol (%) | 2763(56.74) | 1096(55.63) | | | GnRH-a short protocol (%) | 1097(22.53) | 473(24.01) | | | GnRH antagonist (%) | 918(18.85) | 333(16.90) | | | Others (%) | 84(1.72) | 65(3.30) | | | Number of oocytes retrieved | 12.94 ± 5.65 | 13.42 ± 6.18 | 0.009 | | Number of embryos frozen | 3.99 ± 2.65 | 4.09 ± 2.73 | 0.191 | | Endometrial thickness | 1.00 ± 0.17 | 0.90 ± 0.14 | < 0.001 | | Times of previous ET(s) | 0.002 | || | 0 | 1970(40.45) | 799(40.56) | | | 1 | 2251(46.22) | 847(42.99) | | | 2 | 694(14.25) | 324(16.46) | | | Times of previous early miscarriage | 0.030 | || | 0 | 4123(84.66) | 1626(82.54) | | | 1 | 747(15.34) | 344(17.46) | BMI body mass index, AFC antral follicle count, COH controlled ovarian hyperstimulation, Others including mild ovarian stimulation protocol, luteal-phase ovarian stimulation protocol, natural protocol and so on, ET embryo transfer The pregnancy outcomes are shown in Table 2. The clinical pregnancy rate and live birth rate were remarkably higher in NC than that in AC (62.79% vs 57.77%, p < 0.001; 50.02% vs 40.81%, p < 0.001, respectively). However, the early miscarriage rate was significantly lower in NC than in AC (11.81% vs 17.66%, p < 0.001). Table 2. | Natural cycle | Artificial cycle | p-value | | |---|---|---|---| | Clinical pregnancy rate (%) | 62.79 | 57.77 | < 0.001 | | Early miscarriage rate (%) | 11.81 | 17.66 | < 0.001 | | Live birth rate (%) | 50.02 | 40.81 | < 0.001 | Table 3, 4, and 5 show the results of multivariate logistic regression analysis. We found that AC was correlated with a decreased probability of clinical pregnancy rate (aOR 0.852; 95%CI 0.765–0.949, p = 0.004) and live birth rate (aOR 0.746, 95%CI 0.669–0.832, p < 0.001), but an increased probability of early miscarriage rate (aOR 1.447, 95%CI 1.215–1.724, p < 0.001). Table 3. | Crude OR (95% CI) | p-value | Adjusted OR (95% CI) | p-value | || |---|---|---|---|---|---| | Maternal age | 0.954 (0.941–0.967) | < 0.001 | 0.964 (0.951–0.977) | < 0.001 | | | Number of embryos frozen | 1.086 (1.065–1.107) | < 0.001 | 1.070 (1.049–1.092) | < 0.001 | | | Protocol for FET | NC (ref) | 1 | 1 | || | AC | 0.810 (0.729–0.902) | < 0.001 | 0.849 (0.759–0.950) | 0.004 | | | Endometrial thickness | 2.346 (1.740–3.163) | < 0.001 | 1.988 (1.454–2.719) | < 0.001 | | | Times of previous ET(s) | 0 (ref) | 1 | 1 | || | 1 | 0.722 (0.649–0.803) | < 0.001 | 0.799 (0.716–0.892) | < 0.001 | | | 2 | 0.549 (0.474–0.637) | < 0.001 | 0.610 (0.525–0.710) | < 0.001 | Adjusted by maternal age, BMI, regular menstruation, AFC, infertility status, insemination method, protocol for COH, number of oocytes retrieved, number of embryos frozen, protocol for FET, endometrial thickness and times of previous early miscarriage Table 4. | Crude OR (95% CI) | p-value | Adjusted OR (95% CI) | p-value | || |---|---|---|---|---|---| | Number of oocytes retrieved | 0.980 (0.965–0.995) | 0.009 | 0.983 (0.968–0.998) | 0.029 | | | Protocol for FET | NC (ref) | 1 | 1 | || | AC | 1.419 (1.184–1.701) | < 0.001 | 1.436 (1.197–1.723) | < 0.001 | Adjusted by maternal age, BMI, regular menstruation, AFC, infertility status, insemination method, protocol for COH, number of oocytes retrieved, number of embryos frozen, protocol for FET, endometrial thickness and times of previous early miscarriage Table 5. | Crude OR (95% CI) | p-value | Adjusted OR (95% CI) | p-value | || |---|---|---|---|---|---| | Maternal age | 0.954 (0.941–0.966) | < 0.001 | 0.962 (0.949–0.976) | < 0.001 | | | BMI | 0.973 (0.960–0.987) | < 0.001 | 0.985 (0.971–0.999) | 0.039 | | | Number of embryos frozen | 1.081 (1.062–1.101) | < 0.001 | 1.060 (1.040–1.080) | < 0.001 | | | Protocol for FET | NC (ref) | 1 | 1 | || | AC | 0.689 (0.620–0.766) | < 0.001 | 0.737 (0.658–0.825) | < 0.001 | | | Endometrial thickness | 2.779 (2.081–3.711) | < 0.001 | 2.257 (1.659–3.072) | < 0.001 | | | Times of previous ET(s) | 0 (ref) | 1 | 1 | || | 1 | 0.637 (0.574–0.706) | < 0.001 | 0.704 (0.633–0.785) | < 0.001 | | | 2 | 0.478 (0.411–0.555) | < 0.001 | 0.540 (0.462–0.630) | < 0.001 | Adjusted by maternal age, BMI, regular menstruation, AFC, infertility status, insemination method, protocol for COH, number of oocytes retrieved, number of embryos frozen, protocol for FET, endometrial thickness and times of previous early miscarriage A subgroup analysis of women under 35 who underwent first FET cycle (first FET in freezing-all cycles) between two groups showed basically consistent outcomes with the integral outcomes after multivariate logistic regression analysis: clinical pregnancy rate (aOR 0.829, 95%CI 0.689–0.998, p = 0.048) and live birth rate (aOR 0.003, 95%CI 0.637–0.914, p = 0.003), while there was no significant difference in early miscarriage rate between the two groups (p = 0.053).

In this study, we analyzed the pregnancy outcomes of 6840 cycles in non-PCOS patients who underwent VSBT and found that NC could achieve significantly better pregnancy and live birth outcomes than AC. A successful embryo implantation is determined by the quality of embryos and endometrial receptivity. Identification of the “optimal” method for endometrial preparation in an FET cycle is especially important. NC and AC are commonly used methods when preparing the endometrium for implantation [2, 3, 23, 24]. However, the choice of methods for endometrial preparation has been predominantly determined by the presence or absence of regular menstruation, as well as the preference of patients and their physicians. The advantages of AC make it more popular than NC. First, it can avoid unmonitored spontaneous ovulation as in NC. Second, it can reduce the number of clinical visits required by patients for ultrasound scanning and hormone testing. Third, this approach can also help to formulate more flexible embryo transfer plans [25]. However, several recent studies have demonstrated that AC is related to adverse peripartum outcomes including hypertensive disorders of pregnancy and postpartum hemorrhage [15, 26]. Based on these studies, the view of “back to nature” has been espoused [9]. Although there is lack of randomized controlled trials, most reports agreed that NC could achieve better peripartum outcomes than AC [11–16]. However, the comparison of pregnancy outcomes between NC and AC remains controversial among different studies [27–29]. Many randomized controlled trials (RCTs) demonstrate that NC is not superior to AC in clinical pregnancy rate (CPR), early miscarriage rate, and ongoing pregnancy rate (OPR) [20, 30–35], while some cohort studies present different conclusions [11, 19, 36]. There might be several reasons for the controversial conclusions between these studies. Firstly, the clinical practice outcome may change with the development of laboratory technology. In one RCT study reported in 2016, 959 patients underwent slow-freezing embryo transfer after endometrial preparation with AC or a modified NC protocol [17], and the results showed no significant difference in the pregnancy outcomes between the two groups. In this study, the CPR and OPR in both groups were not satisfactory owing to technical limitations, which might have affected the detection of differences between these two endometrial preparation approaches. With the notable improvement of vitrification techniques and blastocyst culture, the average survival rate of vitrified blastocysts reached 98–99% in the current study, and pregnancy outcomes were also improved. Hence, the successful application of blastocyst vitrification makes the comparison between two endometrial preparation protocols more effective and significant. A recent study has supported this viewpoint very well; the researchers found that NC had the highest implantation rate in blastocyst transfer, but showed no advantage in cleavage embryo transfer [37]. Secondly, RCTs are generally considered as the gold standard for testing the efficacy of novel therapeutic interventions. Normally patients are recruited under strict inclusion and exclusion criteria, and the summary results are typically the average treatment effect. However, it might not always be acceptable to make treatment decisions for individual patients based on an overall average treatment effect [38, 39]. Sheikhi et al. conducted an RCT involving vitrification blastocyst transfer and investigated the pregnancy outcomes among NC, ovarian-stimulating cycle, and AC groups [40]. In their study, NC showed a relatively better pregnancy outcome, but no significant difference was found between NC and AC. However, the reliability of their study might have been limited by the low sample size of the NC group, which only included 28 cycles. Few previous studies have performed RCTs to compare the pregnancy outcomes in NC and AC with vitrification blastocyst transfer. We expect that more prospective studies with large-scale and well-controlled design could validate our results. Thirdly, different NC methods to determine the time window for implantation and different hormonal support protocols in AC might influence the comparison results. Prediction of the exact time for embryo transfer is the key issue for successful implantation. In Cerrillo et al., embryo transfer was planned at the 5th–7th day after detection of the urinary LH-surge [41]. While in another study, ovulation was confirmed by transvaginal ultrasound (TVS) [19]. In our center, the combination of TVS and the detection of serum progesterone are used to assure a relatively accurate timing of ovulation. Although using urinary LH surge as a method for detecting ovulation has been verified, one RCT demonstrated that false-positive results occurred in roughly 7% of cycles, which could affect the confirmation of ovulation time [42]. On the other hand, a suboptimal AC protocol might induce inferior clinical outcomes in FET cycles [31]. There is a lack of consensus on guidelines regarding hormonal support protocols in AC, which is another recent focus of concern [32, 33, 43]. From an endocrinological point of view, more hormonal support is required in AC than in NC to compensate for the absence of the corpus luteum. The combined protocol of oral dydrogesterone and vaginal progesterone in the present study is, and has been, the mostly routine luteal support protocol for AC used in our center. Recent literature [44, 45] reports similar progesterone support protocols and found them to compare favorably with other protocols. Whether a more ideal hormonal support protocol in AC could lead to a better outcome is still under investigation [46, 47]. We hope more studies with different practical protocols of endometrial preparation used in NC and AC will address this issue. Our present data from a real-world study with this particular progesterone administration protocol may add to this discussion. Our conclusions are consistent with the results from several other studies [11, 19, 37]. Liu et al. analyzed first FET cycles in 1846 patients, including both cleavage stage embryo and blastocyst transfer, and they found that NC was associated with an increased likelihood of live birth. In another retrospective study of 634 FET cycles, natural ovulatory cycles achieved significantly better pregnancy outcomes than artificial endometrial preparations for FET [48]. Although these clinical data support that NC has a higher pregnancy rate and lower early miscarriage rate, the etiology and pathophysiologic mechanisms are still unclear. Effects from dysfunction of corpora lutea may contribute to this issue. In addition to estrogen and progestogen, corpora lutea also secret relaxin, vascular endothelial growth factor, and other vasoactive substances, which may participate in the implantation and early development of the placenta [13, 26, 49, 50]. Moreover, several studies demonstrate that the endometrial gene expression pattern in NC is similar to that in fertile women. However, the expression of many genes involved in the process of implantation change in AC, which may result from the impact of exogenous estradiol [7, 51, 52]. On the other hand, the administration of exogenous estradiol could increase the risk of thrombosis, which can cause a higher incidence of miscarriage. Some researchers propose a hypothesis that women with abnormal ovulation more commonly proceed with AC, and have higher rates of insulin resistance, which may in turn lead to an unfavorable uterine environment [53]. In general, our study had some advantages compared to other studies. First, our study had a high statistical power owing to a good sample size. Second, we used vitrification single-blastocyst transfer which could avoid the selection bias caused by different cryopreservation techniques and different developmental stages of embryos. Finally, we adjusted a variety of potential confounders to make the results more reliable. Also, we conducted a subgroup analysis of first FET cycles, and achieved outcomes consistent with overall outcomes. However, several limitations were also associated with this study. Firstly, due to the retrospective analysis, patients were assigned to different groups based on the judgements of physicians and the choices of patients. Women with abnormal ovulation more commonly used AC. Therefore, we introduced a variable of regular menstruation into multivariate logistic regression to eliminate this flaw. Secondly, a few patients changed to the AC group after the failure of first FET with NC, which may have resulted in a selection bias. We tried to decrease this bias by introducing the variable of “times of previous ETs” into the analysis. Thirdly, our study did not analyze other factors that may be associated with pregnancy rate, including diet, exercise, and education.

This was a retrospective study based on a sufficient sample size, which investigated the pregnancy outcomes of non-PCOS women who undertook vitrified blastocyst transfer. The findings suggested that NC offer favorable outcomes relative to AC under our present progesterone support protocol. Following the outcome of our present and previous studies [12], NC might be considered as the first-line choice of endometrial preparation protocol in non-PCOS patients, despite the difficulties in confirming ovulation. Further research should address the association of natural corpora lutea with endometrial receptivity, placentation, and maintenance of early gestation. Supplementary Information Below is the link to the electronic supplementary material.

We appreciate all the medical workers and follow-up staff in the Center for Reproductive Medicine Affiliated to Shandong University for their elaborate work. Funding This study was supported by the National Key Research and Development Program of China (2021YFC2700404 and 2018YFC1002804). Declarations Ethics approval The study was approved by the Ethics Committee of Reproductive Medicine of Shandong University. Conflict of interest The authors declare no competing interests. Footnotes Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Contributor Information Shanshan Gao, Email: [email protected]. Jinlong Ma, Email: [email protected].

- 1.Zeilmaker GH, Alberda AT, van Gent I, Rijkmans CM, Drogendijk AC. Two pregnancies following transfer of intact frozen-thawed embryos. Fertil Steril. 1984;42:293–296. doi: 10.1016/s0015-0282(16)48029-5. [DOI] [PubMed] [Google Scholar] - 2.Roque M, Haahr T, Geber S, Esteves SC, Humaidan P. Fresh versus elective frozen embryo transfer in IVF/ICSI cycles: a systematic review and meta-analysis of reproductive outcomes. Hum Reprod Update. 2019;25:2–14. doi: 10.1093/humupd/dmy033. [DOI] [PubMed] [Google Scholar] - 3.Wong KM, van Wely M, Mol F, Repping S, Mastenbroek S. Fresh versus frozen embryo transfers in assisted reproduction. Cochrane Database Syst Rev. 2017;3:CD011184. doi: 10.1002/14651858.CD011184.pub2. [DOI] [PMC free article] [PubMed] [Google Scholar] - 4.Wei D, Liu JY, Sun Y, Shi Y, Zhang B, Liu JQ, Tan J, Liang X, Cao Y, Wang Z, et al. Frozen versus fresh single blastocyst transfer in ovulatory women: a multicentre, randomised controlled trial. Lancet. 2019;393(10178):1310–1318. doi: 10.1016/S0140-6736(18)32843-5. [DOI] [PubMed] [Google Scholar] - 5.Wang N, Zhao X, Ma M, Zhu Q, Wang Y. Effect of day 3 and day 5/6 embryo quality on the reproductive outcomes in the single vitrified embryo transfer cycles. Front Endocrinol (Lausanne) 2021;12:641623. doi: 10.3389/fendo.2021.641623. [DOI] [PMC free article] [PubMed] [Google Scholar] - 6.Miravet-Valenciano JA, Rincon-Bertolin A, Vilella F, Simon C. Understanding and improving endometrial receptivity. Curr Opin Obstet Gynecol. 2015;27:187–192. doi: 10.1097/GCO.0000000000000173. [DOI] [PubMed] [Google Scholar] - 7.Altmäe S, Tamm-Rosenstein K, Esteban FJ, Simm J, Kolberg L, Peterson H, Metsis M, Haldre K, Horcajadas JA, Salumets A, et al. Endometrial transcriptome analysis indicates superiority of natural over artificial cycles in recurrent implantation failure patients undergoing frozen embryo transfer. Reprod Biomed Online. 2016;32:597–613. doi: 10.1016/j.rbmo.2016.03.004. [DOI] [PubMed] [Google Scholar] - 8.Karizbodagh MP, Rashidi B, Sahebkar A, Masoudifar A, Mirzaei H. Implantation window and angiogenesis. J Cell Biochem. 2017;118:41414151. doi: 10.1002/jcb.26088. [DOI] [PubMed] [Google Scholar] - 9.Lawrenz B, Coughlan C, Melado L, Fatemi HM. The ART of frozen embryo transfer: back to nature! Gynecol Endocrinol. 2020;36:479–483. doi: 10.1080/09513590.2020.1740918. [DOI] [PubMed] [Google Scholar] - 10.Casper RF, Yanushpolsky EH. Optimal endometrial preparation for frozen embryo transfer cycles: window of implantation and progesterone support. Fertil Steril. 2016;105:867–872. doi: 10.1016/j.fertnstert.2016.01.006. [DOI] [PubMed] [Google Scholar] - 11.Morozov V, Ruman J, Kenigsberg D, Moodie G, Brenner S. Natural cycle cryo-thaw transfer may improve pregnancy outcome. J Assist Reprod Genet. 2007;24:119–123. doi: 10.1007/s10815-006-9100-y. [DOI] [PMC free article] [PubMed] [Google Scholar] - 12.Wang Z, Liu H, Song H, Li X, Jiang J, Sheng Y, Shi Y. Increased risk of pre-eclampsia after frozen-thawed embryo transfer in programming cycles. Front Med (Lausanne) 2020;7:104. doi: 10.3389/fmed.2020.00104. [DOI] [PMC free article] [PubMed] [Google Scholar] - 13.Zaat TR, Brink AJ, de Bruin JP, Goddijn M, Broekmans FJM, Cohlen BJ, Macklon NS, van Wely M, Groenewoud ER, Mol F, et al. Increased obstetric and neonatal risks in artificial cycles for frozen embryo transfers? Reprod Biomed Online. 2021;42(5):919–929. doi: 10.1016/j.rbmo.2021.01.015. [DOI] [PubMed] [Google Scholar] - 14.von Versen-Höynck F, Häckl S, Selamet Tierney ES, Conrad KP, Baker VL, Winn VD. Maternal Vascular health in pregnancy and postpartum after assisted reproduction. Hypertension. 2020;75:549–560. doi: 10.1161/HYPERTENSIONAHA.119.13779. [DOI] [PMC free article] [PubMed] [Google Scholar] - 15.Singh B, Reschke L, Segars J, Baker VL. Frozen-thawed embryo transfer: the potential importance of the corpus luteum in preventing obstetrical complications. Fertil Steril. 2020;113:252–257. doi: 10.1016/j.fertnstert.2019.12.007. [DOI] [PMC free article] [PubMed] [Google Scholar] - 16.Waschkies F, Kroning L, Schill T, Chandra A, Schippert C, Töpfer D, Ziert Y, von Versen-Höynck F. Pregnancy outcomes after frozen-thawed embryo transfer in the absence of a corpus luteum. Front Med (Lausanne) 2021;8:727753. doi: 10.3389/fmed.2021.727753. [DOI] [PMC free article] [PubMed] [Google Scholar] - 17.Groenewoud ER, Cohlen BJ, Al-Oraiby A, Brinkhuis EA, Broekmans FJ, de Bruin JP, van den Dool G, Fleisher K, Friederich J, Goddijn M, et al. A randomized controlled, non-inferiority trial of modified natural versus artificial cycle for cryo-thawed embryo transfer. Hum Reprod. 2016;31:1483–1492. doi: 10.1093/humrep/dew120. [DOI] [PMC free article] [PubMed] [Google Scholar] - 18.Groenewoud ER, Cohlen BJ, Macklon NS. Programming the endometrium for deferred transfer of cryopreserved embryos: hormone replacement versus modified natural cycles. Fertil Steril. 2018;109:768–774. doi: 10.1016/j.fertnstert.2018.02.135. [DOI] [PubMed] [Google Scholar] - 19.Liu X, Shi W, Shi J. Natural cycle frozen-thawed embryo transfer in young women with regular menstrual cycles increases the live-birth rates compared with hormone replacement treatment: a retrospective cohort study. Fertil Steril. 2020;113(4):811–817. doi: 10.1016/j.fertnstert.2019.11.023. [DOI] [PubMed] [Google Scholar] - 20.Huang F, Qiao J, Liu J, Chen Z, Cao Y, Wu L, Jin F, Lin G, Liu P, Zhu Y, et al. Chinese Expert Consensus Compilation Group for preimplantation genetic diagnosis/screening. Chin J Obstet Gynecol. 2018;35:151–155. doi: 10.3760/cma.j.issn.1003-9406.2018.02.001. [DOI] [Google Scholar] - 21.Chen ZJ, Zhang YW, Liu JY, et al. [China diagnosis criteria of polycystic ovarian syndrome diagnosis] Chin J Obstet Gynecol. 2012;47:74–5. doi: 10.3760/cma.j.issn.0529-567x.2012.01.022. [DOI] [Google Scholar] - 22.Chen ZJ, Shi Y, Sun Y, Zhang B, Liang X, Cao Y, Yang J, Liu J, Wei D, Weng N, et al. Fresh versus frozen embryos for infertility in the polycystic ovary syndrome. N Engl J Med. 2016;375:523–533. doi: 10.1056/NEJMoa1513873. [DOI] [PubMed] [Google Scholar] - 23.Shi Y, Sun Y, Hao C, Zhang H, Wei D, Zhang Y, Zhu Y, Deng X, Qi X, Li H, et al. Transfer of fresh versus frozen embryos in ovulatory women. N Engl J Med. 2018;378:126–136. doi: 10.1056/NEJMoa1705334. [DOI] [PubMed] [Google Scholar] - 24.Michelmann HW, Nayudu P. Cryopreservation of human embryos. Cell Tissue Bank. 2006;7:135–141. doi: 10.1007/s10561-005-0877-1. [DOI] [PubMed] [Google Scholar] - 25.Weissman A, Levin D, Ravhon A, Eran H, Golan A, Levran D. What is the preferred method for timing natural cycle frozen-thawed embryo transfer? Reprod Biomed Online. 2009;19:66–71. doi: 10.1016/s1472-6483(10)60048-x. [DOI] [PubMed] [Google Scholar] - 26.Ginström Ernstad E, Wennerholm UB, Khatibi A, Petzold M, Bergh C. Neonatal and maternal outcome after frozen embryo transfer: increased risks in programmed cycles. Am J Obstet Gynecol. 2019;221(126):e1–126.e18. doi: 10.1016/j.ajog.2019.03.010. [DOI] [PubMed] [Google Scholar] - 27.Kalem Z, NamlıKalem M, Bakırarar B, Kent E, Gurgan T. Natural cycle versus hormone replacement therapy cycle in frozen-thawed embryo transfer. Saudi Med J. 2018;39:1102–1108. doi: 10.15537/smj.2018.11.23299. [DOI] [PMC free article] [PubMed] [Google Scholar] - 28.Poletto KQ, Lobo MP, Giovanucci M, Approbato MS, Castro EC. Pregnancy rates from natural and artificial cycles of women submitted to frozen embryo transfers: a metanalysis. JBRA Assist Reprod. 2019;23:268–272. doi: 10.5935/1518-0557.20190018. [DOI] [PMC free article] [PubMed] [Google Scholar] - 29.Glujovsky D, Pesce R, Sueldo C, QuinteiroRetamar AM, Hart RJ, Ciapponi A. Endometrial preparation for women undergoing embryo transfer with frozen embryos or embryos derived from donor oocytes. Cochrane Database Syst Rev. 2020;10:CD006359. doi: 10.1002/14651858.CD006359.pub3. [DOI] [PMC free article] [PubMed] [Google Scholar] - 30.Sheikhi O, Golsorkhtabaramiri M, Esmaeilzadeh S, Mahouti T, Heidari FN. Reproductive outcomes of vitrified blastocyst transfer in modified natural cycle versus mild hormonally stimulated and artificial protocols: a randomized control trial. JBRA Assist Reprod. 2018;22:221–227. doi: 10.5935/1518-0557.20180040. [DOI] [PMC free article] [PubMed] [Google Scholar] - 31.Devine K, Richter KS, Widra EA, McKeeby JL. Vitrified blastocyst transfer cycles with the use of only vaginal progesterone replacement with endometrin have inferior ongoing pregnancy rates: results from the planned interim analysis of a three-arm randomized controlled noninferiority trial. Fertil Steril. 2018;109:266–275. doi: 10.1016/j.fertnstert.2017.11.004. [DOI] [PubMed] [Google Scholar] - 32.Toth TL, Vaughan DA. Optimizing luteal support in frozen embryo transfer cycles. Fertil Steril. 2018;109:242–243. doi: 10.1016/j.fertnstert.2017.12.008. [DOI] [PubMed] [Google Scholar] - 33.Manno M, Tomei F, Fuggetta G, Piccolo S. Artificial cycle ‘per se’ or the specific protocol of endometrial preparation as responsible for obstetric complications of frozen cycle? Hum Reprod. 2019;34:2553–2554. doi: 10.1093/humrep/dez220. [DOI] [PubMed] [Google Scholar] - 34.Madani T, Ramezanali F, Yahyaei A, Hasani F, Bagheri Lankarani N, Mohammadi YL. Live birth rates after different endometrial preparation methods in frozen cleavage-stage embryo transfer cycles: a randomized controlled trial. Arch Gynecol Obstet. 2019;299:1185–1191. doi: 10.1007/s00404-019-05062-7. [DOI] [PubMed] [Google Scholar] - 35.Agha-Hosseini M, Hashemi L, Aleyasin A, Ghasemi M, Sarvi F, Shabani Nashtaei M, Khodarahmian M. Natural cycle versus artificial cycle in frozen-thawed embryo transfer: a randomized prospective trial. Turk J Obstet Gynecol. 2018;15:12–17. doi: 10.4274/tjod.47855. [DOI] [PMC free article] [PubMed] [Google Scholar] - 36.Chang EM, Han JE, Kim YS, Lyu SW, Lee WS, Yoon TK. Use of the natural cycle and vitrification thawed blastocyst transfer results in better in-vitro fertilization outcomes: cycle regimens of vitrification thawed blastocyst transfer. J Assist Reprod Genet. 2011;28:369–374. doi: 10.1007/s10815-010-9530-4. [DOI] [PMC free article] [PubMed] [Google Scholar] - 37.Cardenas Armas DF, Peñarrubia J, Goday A, Guimerá M, Vidal E, Manau D, Fabregues F. Frozen-thawed blastocyst transfer in natural cycle increase implantation rates compared artificial cycle. Gynecol Endocrinol. 2019;35:873–877. doi: 10.1080/09513590.2019.1600668. [DOI] [PubMed] [Google Scholar] - 38.Takahashi K, Serruys PW, Fuster V, Farkouh ME, Spertus JA, Cohen DJ, Park SJ, Park DW, Ahn JM, Kappetein AP, et al. Redevelopment and validation of the SYNTAX score II to individualise decision making between percutaneous and surgical revascularisation in patients with complex coronary artery disease: secondary analysis of the multicentre randomised controlled SYNTAXES trial with external cohort validation. Lancet. 2020;396(10260):1399–1412. doi: 10.1016/S0140-6736(20)32114-0. [DOI] [PubMed] [Google Scholar] - 39.Fanaroff AC, Califf RM, Harrington RA, Granger CB, McMurray JJV, Patel MR, Bhatt DL, Windecker S, Hernandez AF, Gibson CM, et al. Randomized trials versus common sense and clinical observation: JACC review topic of the week. J Am Coll Cardiol. 2020;76:580–589. doi: 10.1016/j.jacc.2020.05.069. [DOI] [PMC free article] [PubMed] [Google Scholar] - 40.Mounce G, McVeigh E, Turner K, Child TJ. Randomized, controlled pilot trial of natural versus hormone replacement therapy cycles in frozen embryo replacement in vitro fertilization. Fertil Steril. 2015;104:915–920.e1. doi: 10.1016/j.fertnstert.2015.07.1131. [DOI] [PubMed] [Google Scholar] - 41.Cerrillo M, Herrero L, Guillén A, Mayoral M, García-Velasco JA. Impact of endometrial preparation protocols for frozen embryo transfer on live birth rates. Rambam Maimonides Med J. 2017;8:e0020. doi: 10.5041/RMMJ.10297. [DOI] [PMC free article] [PubMed] [Google Scholar] - 42.McGovern PG, Myers ER, Silva S, Coutifaris C, Carson SA, Legro RS, Schlaff WD, Carr BR, Steinkampf MP, Giudice LC, et al. Absence of secretory endometrium after false-positive home urine luteinizing hormone testing. Fertil Steril. 2004;82:1273–1277. doi: 10.1016/j.fertnstert.2004.03.070. [DOI] [PubMed] [Google Scholar] - 43.Saito K, Kuwahara A, Ishikawa T, Morisaki N, Miyado M, Miyado K, Fukami M, Miyasaka N, Ishihara O, Irahara M, et al. Reply: artificial cycle ‘per se’ or the specific protocol of endometrial preparation as responsible for obstetric complications of frozen cycle? Hum Reprod. 2019;34:2554–2555. doi: 10.1093/humrep/dez221. [DOI] [PubMed] [Google Scholar] - 44.Vuong LN, Nguyen LK, Le AH, Pham HH, Ho VN, Le HL, Pham TD, Dang VQ, Phung TH, Smitz J, et al. Fresh embryo transfer versus freeze-only after in vitro maturation with a pre-maturation step in women with high antral follicle count: a randomized controlled pilot study. J Assist Reprod Genet. 2021;38:1293–1302. doi: 10.1007/s10815-021-02180-7. [DOI] [PMC free article] [PubMed] [Google Scholar] - 45.Deng Y, Ou Z, Liang Y, Chen Z. Sun L [Efficacy of vaginal administration of Crinone versus utrogestan combined with oral dydrogesterone tablets for luteal support in PGT freeze-thaw embryo transfer cycles] J South Med Univ. 2021;41:1062–1066 . doi: 10.12122/j.issn.1673-4254.2021.07.14. [DOI] [PMC free article] [PubMed] [Google Scholar] - 46.Volovsky M, Pakes C, Rozen G, Polyakov A. Do serum progesterone levels on day of embryo transfer influence pregnancy outcomes in artificial frozen-thaw cycles? J Assist Reprod Genet. 2020;37:1129–1135. doi: 10.1007/s10815-020-01713-w. [DOI] [PMC free article] [PubMed] [Google Scholar] - 47.Gao H, Ye J, Ye H, Hong Q, Sun L, Chen Q. Strengthened luteal phase support for patients with low serum progesterone on the day of frozen embryo transfer in artificial endometrial preparation cycles: a large-sample retrospective trial. Reprod Biol Endocrinol. 2021;19:60. doi: 10.1186/s12958-021-00747-8. [DOI] [PMC free article] [PubMed] [Google Scholar] - 48.Aslih N, Dorzia D, Atzmon Y, Estrada D, Ellenbogen A, Bilgory A, Shalom-Paz E. Ovulatory-based FET cycles may achieve higher pregnancy rates in the general population and among anovulatory women. J Clin Med. 2021;10:703. doi: 10.3390/jcm10040703. [DOI] [PMC free article] [PubMed] [Google Scholar] - 49.Conrad KP. Emerging role of relaxin in the maternal adaptations to normal pregnancy: implications for preeclampsia. Semin Nephrol. 2011;31:15–32. doi: 10.1016/j.semnephrol.2010.10.003. [DOI] [PMC free article] [PubMed] [Google Scholar] - 50.Conrad KP, Baker VL. Corpus luteal contribution to maternal pregnancy physiology and outcomes in assisted reproductive technologies. Am J Physiol Regul Integr Comp Physiol. 2013;304:R69–72. doi: 10.1152/ajpregu.00239.2012. [DOI] [PMC free article] [PubMed] [Google Scholar] - 51.Ma WG, Song H, Das SK, Paria BC, Dey SK. Estrogen is a critical determinant that specifies the duration of the window of uterine receptivity for implantation. Proc Natl Acad Sci USA. 2003;100:2963–2968. doi: 10.1073/pnas.0530162100. [DOI] [PMC free article] [PubMed] [Google Scholar] - 52.Fritz R, Jindal S, Feil H, Buyuk E. Elevated serum estradiol levels in artificial autologous frozen embryo transfer cycles negatively impact ongoing pregnancy and live birth rates. J Assist Reprod Genet. 2017;34:1633–1638. doi: 10.1007/s10815-017-1016-1. [DOI] [PMC free article] [PubMed] [Google Scholar] - 53.Veleva Z, Orava M, Nuojua-Huttunen S, Tapanainen JS, Martikainen H. Factors affecting the outcome of frozen-thawed embryo transfer. Hum Reprod. 2013;28:2425–2431. doi: 10.1093/humrep/det251. [DOI] [PubMed] [Google Scholar] Associated Data This section collects any data citations, data availability statements, or supplementary materials included in this article.