Fresh embryo transfer versus frozen embryo transfer in in vitro fertilization cycles: a systematic review and meta-analysis

Transcription

1 Fresh embryo transfer versus frozen embryo transfer in in vitro fertilization cycles: a systematic review and meta-analysis Matheus Roque, M.D., a,c Karinna Lattes, M.D., a,d Sandra Serra, M.Sc., a,d Ivan Sola, B.Psych., e,f,g Selmo Geber, Ph.D., c,h Ramon Carreras, Ph.D., b and Miguel Angel Checa, Ph.D. b,d a Master Internacional Medicina Reproductiva, Hospital del Mar, and b Department of Obstetrics and Gynecology, Parc de Salut Mar, Universitat Autonoma de Barcelona, Barcelona, Spain; c Origen Center for Reproductive Medicine, Belo Horizonte, Brazil; d Centro de Infertilidad y Reproduccion Humana, Barcelona, Spain; e Iberoamerican Cochrane Center, Barcelona, Spain; f Institute of Biomedical Research (IIB Sant Pau), Barcelona, Spain; g CIBER Epidemiología y Salud Publica, Barcelona, Spain; and h Universidade Federal de Minas Gerais, Belo Horizonte, Brazil Objective: To examine the available evidence to assess if cryopreservation of all embryos and subsequent frozen embryo transfer (FET) results in better outcomes compared with fresh transfer. Design: Systematic review and meta-analysis. Setting: Centers for reproductive care. Patient(s): Infertility patient(s). Intervention(s): An exhaustive electronic literature search in MEDLINE, EMBASE, and the Cochrane Library was performed through December We included randomized clinical trials comparing outcomes of IVF cycles between fresh and frozen embryo transfers. Main Outcome Measure(s): The outcomes of interest were ongoing pregnancy rate, clinical pregnancy rate, and miscarriage. Result(s): We included three trials accounting for 633 cycles in women aged years. Data analysis showed that FET resulted in significantly higher ongoing pregnancy rates and clinical pregnancy rates. Conclusion(s): Our results suggest that there is evidence that IVF outcomes may be improved by performing FET compared with fresh embryo transfer. This could be explained by a better embryo-endometrium synchrony achieved with endometrium preparation cycles. (Fertil Steril Ò 2013;99: Ó2013 by American Society for Reproductive Medicine.) Key Words: Fresh embryo transfer, frozen embryo transfer, endometrial receptivity, pregnancy outcome Discuss: You can discuss this article with its authors and with other ASRM members at fertstertforum.com/roquem-fresh-versus-frozen-embryo-transfer/ Use your smartphone to scan this QR code and connect to the discussion forum for this article now.* * Download a free QR code scanner by searching for QR scanner in your smartphone s app store or app marketplace. Implantation represents one of the important steps for the success of assisted reproduction techniques (ART) (1). Its effectiveness relies on three main parameters: embryo quality, endometrial receptivity (ER), and a well-balanced embryoendometrium interaction (2). The implantation window is a self-limited period in which the endometrium has acquired the adequate morphologic and functional state for the blastocyst attachment. Therefore, ER is essential for conception in natural and infertility treatment cycles. However, it has Received May 30, 2012; revised August 31, 2012; accepted September 5, 2012; published online October 3, M.R. has nothing to disclose. K.L. has nothing to disclose. S.S. has nothing to disclose. I.S. has nothing to disclose. S.G. has nothing to disclose. R.C. has nothing to disclose. M.A.C. has nothing to disclose. Reprint requests: Miguel Angel Checa, Ph.D., Department of Obstetrics and Gynecology, Parc de Salut Mar, PasseigMarítim 25 29, E Barcelona, Spain ( macheca@parcdesalutmar.cat). Fertility and Sterility Vol. 99, No. 1, January /$36.00 Copyright 2013 American Society for Reproductive Medicine, Published by Elsevier Inc. been suggested that controlled ovarian hyperstimulation (COH) adversely affects ER during ART cycles (3 5). This interaction is mediated by the supraphysiologic levels of estradiol (E 2 ) and progesterone (P) during the follicular phase, leading to morphologic and biochemical endometrial alterations and a more advanced endometrium than in natural cycles. Ultimately, these physiologic changes may affect the success rates of the treatments. These altered hormone levels could mediate an asynchrony between the endometrium and the transferred embryos, leading to an endometrial 156 VOL. 99 NO. 1 / JANUARY 2013

2 Fertility and Sterility environment that could be responsible for implantation failure (6 10). In ART, the highest pregnancy rates are obtained in fresh oocyte donation cycles. In these cycles, the endometrium is artificially primed and the embryos are therefore transferred to an environment that had not suffered the effects of the supraphysiologic hormonal levels that occur during COH (8). Although the oocytes are of the same quality, some studies of shared oocyte cycles found significantly higher pregnancy rates in recipients compared with oocyte donors, and this may be related to a superior quality of ER (7, 8). Similarly, in frozen embryo transfers (FET), endometrial priming may be achieved with the use of E 2 and P, and the endometrial development can be controlled more precisely than in cycles of COH with gonadotropins (9, 10). To date, with the advances of the embryo cryopreservation techniques, the quality of the frozen embryos and their potential of implantation are similar to the observed with fresh embryos (11, 12). Although in most of the studies comparing fresh and FET, the best-quality embryos are chosen for the fresh transfer, and the results are similar between the two types of treatments (13). Some studies have shown good results with the cryopreservation of all embryos and subsequent FET in patients with an increased risk ovarian hyperstimulation syndrome (OHSS) (14 16). Therefore, if the best-quality embryos are selected for FET and the ER can be improved in these cycles, one can expect to obtain higher implantation rates, thus improving overall ART success. The purpose of the present systematic review and metaanalysis was to examine the literature and identify results of randomized clinical trials to assess if the cryopreservation of all embryos of good quality, and subsequent FET, is associated with improvements in the ART outcomes compared with fresh embryo transfer. MATERIALS AND METHODS Given that this was a meta-analysis and did not involve any intervention in humans, the present study did not require the approval of an International Review Board. We used the Preferred Outcome Items for Systematic Reviews and Metaanalysis (PRISMA statement) to report the results of this systematic review (17). Search Strategy An exhaustive electronic search was performed with the MEDLINE AND EMBASE databases, as well as the Cochrane Central Register of Controlled Trials, from their inceptions through December We also searched the main ongoing clinical trial registries, including controlled-trials.com, clinicaltrials.gov, and the International Clinical Trials Registry Platform from the World Health Organization. We used relevant terms and related variants for the interventions and population study: in vitro fertilization with or without intracytoplasmic sperm injection (ICSI) and fresh or frozen embryo transfer. We restricted the search to articles published in English. The search strategy was modified to fit with the syntaxes required in each database. We also searched among the references of the relevant articles. Eligibility Criteria and Data Extraction We included randomized controlled trials (RCTs) of couples undergoing in vitro fertilization (IVF), with or without intracytoplasmic sperm injection (ICSI), that compared the ART outcomes of fresh versus elective frozen embryo transfer. In a first screening, two independent authors (K.L.A., S.S.T.) assessed all of the abstracts retrieved from the search, and then they obtained the full manuscripts of citations that fit the inclusion criteria. They evaluated the studies, eligibility, quality, and extracted data, and any discrepancies were resolved by mutual agreement and, if needed, by reaching a consensus with a third author (M.A.C.). Inter-rater agreement was analyzed with the use of weighted kappa for the inclusion criteria. Outcome Measures The outcomes of interest for this systematic review included the following variables: ongoing pregnancy rate (per woman randomized), clinical pregnancy rate (per woman randomized), and miscarriage rates (per woman randomized). Ongoing pregnancy was defined as pregnancy proceeding beyond the 10th gestational week. Clinical pregnancy was defined as the observation of intrauterine fetal heart motion by 7 weeks' gestation. Miscarriage included any pregnancies that did not become ongoing pregnancies. Risk of Bias Assessment We assessed the risk of bias from included studies following the guidance suggested by the Cochrane Collaboration (18). We made explicit judgment regarding the generation of sequence allocation, allocation concealment, blinding, and incomplete outcome data for each trial included in the review. A judgment of yes for all domains indicates a low risk for bias, whereas a judgment of no for one or more domains indicates a high risk of bias. An unclear judgment in any domain indicates an unclear risk of bias. Analysis For each study, the treatment effect was measured with risk ratios (RRs) for dichotomous outcomes, presented with their corresponding 95% confidence intervals (CIs). We extracted event data following the intention-to-treat principle. Statistical significance was set at a P value of <.05. When possible, we pooled outcome data from each study with the use of a Mantel-Haenszel model, applying the fixed-effects model. We quantified statistical heterogeneity with the use of the I 2 statistic to describe variation across trials that is due to heterogeneity and not to sampling error (19). We used the Review Manager 5 software for conducting the meta-analysis. RESULTS Our electronic search retrieved 64 articles. After the screening of titles and abstracts, we determined that 46 of them were not RCTs, 56 of them did not include the objectives of our metaanalysis, and 52 of them did not inform on the outcomes of interest, leaving seven articles considered to be eligible by one or both reviewers. Among these, in a second selection, VOL. 99 NO. 1 / JANUARY

3 ORIGINAL ARTICLE: ASSISTED REPRODUCTION four articles were excluded because they were not RCTs (20 23). The two authors had good agreement on the selection of trials for inclusion (weighted kappa 0.78, 95% CI ). The complete selection process is depicted in Figure 1. Description of Included Studies Three RCTs assessing the outcome of fresh versus FET in women undergoing IVF (with or without ICSI) met the inclusion criteria, and all reported data on the outcomes analyzed in this review are based on those studies. Overall, the three included studies account for 633 ART cycles in women, with ages ranging from 27 to 33 years. One of the trials (13) included normal responders submitted to ovarian stimulation with recombinant FSH and GnRH antagonist for pituitary suppression. The other two studies included high responders (24, 25). In one of them (24), the ovarian stimulation was performed with GnRH agonist for pituitary suppression and recombinant FSH, and in the other study (25) with recombinant FSH and GnRH antagonist for pituitary suppression. The endometrial priming in the FET group was performed with either oral E 2 and intramuscular P (24) along with leuprolide acetate for pituitary down-regulation, or oral E 2 and E 2 patches as needed, and intramuscular P (13, 25). The characteristics of the studies included in the review are presented in Table 1. Internal Validity of Included Studies The internal validity of the included trials was not threatened due to major sources of bias, although the second Shapiro et al. study (25) was published as correspondence and it was not possible to assess the complete data. The other included trials had properly randomized their participants and had concealed the randomization allocations by means of sealed opaque envelopes. The included studies showed sufficient details to determine whether the outcome assessors were blinded (Table 2). Outcomes All of the trials included in the review contributed to the pooled analyses for the considered outcomes. The analysis of the data available from the three included trials, including FIGURE 1 Preferred Outcome Items for Systematic Reviews and Meta-analysis flow diagram detailing selection of studies for inclusion. RCT ¼ randomized controlled study. 158 VOL. 99 NO. 1 / JANUARY 2013

4 Fertility and Sterility TABLE 1 Characteristics of the clinical trial included in the review. Study ID Patients (Fresh/FET) Age, y (Fresh/FET) Duration of trial Aflatoonian et al. (24) 374 (187/187) High responders Shapiro et al. (13) 137 (67/70) Normal responders Shapiro et al. (25) 122 (62/60) High responders / February 2007 February / October 2007 October 2010 Day of embryo transfer Day 2 Day 5 (blastocyst) Outcome Ongoing pregnancy Implantation Clinical pregnancy Miscarriage rate Ongoing pregnancy Implantation Clinical pregnancy Early pregnancy loss / July 2007 July 2010 Day 5 (blastocyst) Ongoing pregnancy Implantation Clinical pregnancy Early pregnancy loss 263 events, showed that FET resulted in a statistically significant increase in the ongoing pregnancy rate compared with the rate observed with fresh transfer (RR 1.32, 95% CI ; I 2 ¼ 0; Fig. 2A). This pattern of results was also observed for the rates of clinical pregnancies, which was higher in the women allocated in the FET group (280 events; RR 1.31, 95% CI ; I 2 ¼ 0; Fig. 2B). Finally, the fresh group showed a higher miscarriage rate compared with the FET group, but this difference did not reach statistical significance (33 events; RR 0.83, 95% CI ; I 2 ¼ 0; Fig. 2C). DISCUSSION This systematic review showed that the use of FET, compared with fresh embryo transfer, significantly improved clinical and ongoing pregnancy rates, in patients submitted to ART. To our knowledge, this is the first comprehensive review consisting of a pooled analysis that has addressed the question of whether the cryopreservation of all viable embryos and subsequent FET is associated with improved ART outcomes compared with fresh embryo transfer. The results of this meta-analysis suggest that, in normal- and high-responder patients, it may be advantageous to cryopreserve all viable embryos and use them in a subsequent FET. Importantly, the data were extracted to allow for an intention-to-treat analysis. The results favoring FET instead of fresh embryo transfer may be related to the adverse effects of COH on endometrial receptivity, as well as the improved results that can be achieved with current cryopreservation methods (13, 24, 25). The quality of the available evidence that supports these results is moderate (26), and the main limitation of the available evidence is that most of the estimates of the outcomes of interest are based on few events and a type 1 error cannot be ruled out. Embryo implantation is one of the important steps for reproductive success, and implantation failure remains an unsolved problem in ART. In two-thirds of the implantation failures, the primary responsible source of failure is the impairment of the ER, whereas the embryo itself is responsible for only one-third of the failures (2). At the end of the follicular phase in COH, the subtle increases in serum P levels (i.e., premature luteinization) show a positive correlation with FSH levels, and this increase is associated with an advanced endometrial ultrastructural morphology and echogenicity (9, 27 33). In fresh cycles with COH, the elevated P may lead to advanced endometrial maturation, without affecting the quality of the embryo, and this may cause decreased TABLE 2 Quality assessment of included trials. Study (reference) Explicit eligibility criteria Sequence generation Aflatoonian et al. (24) Yes Yes Computer-generated sequence Shapiro et al. (13) Yes Yes Drawing of envelopes Allocation concealed Yes Sealed opaque envelopes Yes Sealed opaque unmarked envelopes Patient blinding No Open trial No Open trial Outcome assessor blinding Yes Unclear Unclear Shapiro et al. (25) Unclear Unclear Unclear Unclear Unclear Unclear Patient follow-up 137 women randomized, 103 analyzed (75%). In the rest the transfer was canceled (in 15% for no viable embryos) VOL. 99 NO. 1 / JANUARY

5 ORIGINAL ARTICLE: ASSISTED REPRODUCTION FIGURE 2 (A C) Meta-analysis results. implantation rates owing to asynchrony between embryo and the endometrium in fresh cycles (3, 34). Uterine receptivity is better achieved during natural cycles or with hormone replacement therapy with exogenous E 2 and P, compared with stimulated cycles (7, 35, 36). There is evidence showing that high E 2 levels (>2,500 pg/ml) may impair the endometrium maturation and implantation (37, 38). Several studies have described an advanced endometrium in the early luteal phase of women submitted to COH, and that if this advancement exceeded 3 days, no pregnancy was observed (3 5, 30, 39, 40). The gene expression profile of human endometrium may be over- or underexpressed in patients undergoing COH, and the expression of E 2 and P receptors is altered in stimulated cycles, indicating an advance in the endometrium maturation compared with natural cycles (38, 41 44). The cryopreservation of embryos has become a routine procedure in ART when embryo transfer is either impossible or inconvenient. When the FET is performed, the endometrial preparation may be achieved in a natural or an artificial way. It is suggested that during the endometrial priming for FET, the endometrium is more receptive than in fresh embryo cycles (45 49). There are different ways to perform the endometrial preparation, but there is a lack of evidence to recommend any one particular protocol for endometrial priming regarding ART outcomes after FET (50). The recent use of vitrification technique has shown a higher embryo survival rate, compared with slow freezing, resulting in significantly higher implantation and pregnancy rates per transfer (45, 51 57). Therefore, the use of elective cryopreservation of viable embryos could be an alternative to avoid the deleterious effects of the COH in embryoendometrium synchrony (13, 20, 25). Although the three studies included in the present metaanalysis did not evaluate live birth rates, earlier conclusions on treatment effects based solely on the clinical and ongoing pregnancy rates as the main outcomes are generally accepted (23), and these variables are comparable to live birth as a measure of efficacy (13, 58). In all of the studies, the endometrial receptivity could be inferred only, not directly assessed. It is possible that the frozen-thaw process may have indirectly involved the selection of the best embryos by improving the proportion of good embryos in the FET group, thereby overestimating the effects attributed to the better endometrial 160 VOL. 99 NO. 1 / JANUARY 2013

6 Fertility and Sterility quality in FET (13). Because all the trials in this meta-analysis included potentially normal- and high-responder patients, these results should not be extrapolated to all types of patients submitted to ART. In summary, the results of this meta-analysis suggest that there is evidence of moderate quality that the implantation, clinical, and ongoing pregnancy rates of ART cycles may be improved by performing FET compared with fresh embryo transfer. These results may be explained by improved embryo-endometrium synchrony achieved with endometrium preparation cycles instead of COH cycles. The data provide a rationale for conducting further randomized clinical trials and multicenter studies to assess the consequences of COH on endometrial receptivity and ART outcomes as they relate to the FET. Acknowledgments: The authors thank Ara Cantillo and Susan García for their collaboration with literature searches, retrieving identified trials, and scientific advice. REFERENCES 1. Díaz-Gimeno P, Horcajadas JA, Martínez-Conejero JA, Esteban FJ, AlamaP, Pellicer A, et al. A genomic diagnostic tool for human endometrial receptivity based on the transcriptomic signature. Fertil Steril 2011;95: Achache H, Revel A. Endometrial receptivity markers, the journey to successful embryo implantation. Hum Reprod Update 2006;12: Bourgain C, Devroey P. The endometrium in stimulated cycles for IVF. Hum Reprod Update 2003;9: Devroey P, Bourgain C, Macklon NS, Fauser BC. Reproductive biology and IVF: ovarian stimulation and endometrial receptivity. Trends Endocrinol Metab 2004;15: Kolibianakis E, Bourgain C, Albano C, Osmanagaoglu K, Smitz J, Steirteghem AV, et al. Effect of ovarian stimulation with recombinant follicle-stimulating hormone, gonadotropin release hormone antagonists, and human chorionic gonadotropin on endometrial maturation on the day of oocyte pick-up. Fertil Steril 2002;78: Nikas G, Develioglu OH, Toner JP, Jones HW Jr. Endometrial pinopodes indicate a shift in the window of receptivity in IVF cycles. Hum Reprod 1999;14: Simon C, Velasco JJG, Valbuena D, Peinado JA, Moreno C, Remohí J, et al. Increasing uterine receptivity by decreasing estradiol levels during the preimplantation period in high responders with the use of a follicle-stimulating hormone step-down regimen. Fertil Steril 1998;70: Shapiro BS, Daneshmand ST, Garner FC, Aguirre M, Hudson C, Thomas S. High ongoing pregnancy rates after deferred transfer through bipronuclear oocyte cryopreservation and post-thaw extended culture. Fertil Steril 2009; 92: Shapiro BS, Daneshmand ST, Garner FC, Aguire M, Thomas S. Large blastocyst diameter, early blastulation, and low preovulatory serum progesterone are dominant predictors of clinical pregnancy in fresh autologous cycles. Fertil Steril 2008;90: Richter KS, Shipley SK, McVearry I, Tucker MJ, Widra EA. Cryopreserved embryo transfers suggest that endometrial receptivity may contribute to reduced success rates of later developing embryos. Fertil Steril 2006;86: Herrero L, Martínez M, Garcia-Velasco JA. Current status of human oocyte and embryo cryopreservation. Curr Opin ObstetGynecol 2011;23: Shapiro BS, Daneshmand ST, Garner FC, Aguirre M, Hudson C, Thomas S. Similar ongoing pregnancy rates after blastocyst transfer in fresh donor cycles and autologous cycles using cryopreserved bipronuclear oocytes suggest similar viability of transferred blastocysts. Fertil Steril 2010;93: Shapiro BS, Daneshmand ST, Garner FC, Aguirre M, Hudson C, Thomas S. Evidence of impaired endometrial receptivity after ovarian stimulation for in vitro fertilization: a prospective randomized trial comparing fresh and frozen-thawed embryo transfer in normal responders. Fertil Steril 2011; 96: Griesinger G, von Otte S, Schroer A, Ludwig AK, Diedrich K, Al-Hasani S, et al. Elective cryopreservation of all pronuclear oocytes after GnRH agonist triggering of final oocyte maturation in patients at risk of developing OHSS: a prospective, observational proof-of-concept study. Hum Reprod 2007;22: d'angelo A. Ovarian hyperstimulation syndrome prevention strategies: cryopreservation of all embryos. Semin Reprod Med 2010;28: Griesinger G, Schultz L, Bauer T, Broessner A, Frambach T, Kissler S. Ovarian hyperstimulation syndrome prevention by gonadotropin-releasing hormone agonist triggering of final oocyte maturation in a gonadotropin-releasing hormone antagonist protocol in combination with freeze-all strategy: a prospective multicentric study. Fertil Steril 2011;95: Moher D, Liberati A, Tetzlaff J, Altman DG. Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. BMJ 2009; 339: Higgins JPT, Green S. Cochrane handbook for systematic reviews of interventions version London: Cochrane Collaboration; Higgins JP, Thompson SG, Deeks JJ, Altman DG. Measuring inconsistency in meta-analyses. BMJ 2003;327: Aflatoonian A, Moghaddam FM, Mashayekhy M, Mohamadian F. Comparison of early pregnancy and neonatal outcomes after frozen and fresh embryo transfer in ART cycles. J Assist Reprod Genet 2010;27: Wang YA, Costello M, Chapman M, Black D, Sullivan EA. Transfers of fresh blastocysts and blastocysts cultured from thawed cleavage embryos are associated with fewer miscarriages. Reprod Biomed Online 2011;23: Zhou F, Lin XN, Tong XM, Li C, Liu L, Jin XY, et al. A frozen-thawed embryo transfer program improves the embryo utilization rate. Chin Med J (Engl) 2009;122: Check JH, Wilson C, Choe JK, Amui J, Katsoff B. A comparison of pregnancy rates following fresh and frozen embryo transfer according to the use of leuprolide acetate vs ganirelix vs cetrorelix. Clin Exp Obstet Gynecol 2010;37: Aflatoonian A, Oskouian H, Ahmadi S, Oskouian L. Can fresh embryo transfers be replaced by cryopreserved-thawed embryo transfers in assisted reproductive cycles? A randomized controlled trial. J Assist Reprod Genet 2010;27: Shapiro BS, Daneshmand ST, Garner FC, Aguirre M, Hudson C, Thomas S. Evidence of impaired endometrial receptivity after ovarian stimulation for in vitro fertilization: a prospective randomized trial comparing fresh and frozen-thawed embryo transfers in high responders. Fertil Steril 2011;96: Guyatt GH, Oxman AD, Vist GE, Kunz R, Falck-Ytter Y, et al, GRADE Working Group. GRADE: an emerging consensus on rating quality of evidence and strength of recommendations. BMJ 2008;336: Shapiro BS, Daneshmand ST, Garner FC, Aguirre M, Hudson C, Thomas S. Embryo cryopreservation rescues cycles with premature luteinization. Fertil Steril 2010;93: Al-Azemi M, Kyrou D, Kolibianakis EM, Humaidan P, van Vaerenbergh I, Devroey P, Fatemi HM. Elevated progesterone during ovarian stimulation for IVF. Reprod Biomed Online 2012;24: Venetis CA, Kolibianakis EM, Papanikolaou E, Bontis J, Devroey P, Tarlatzis BC. Is progesterone elevation on the day of human chorionic gonadotrophin administration associated with the probability of pregnancy in in vitro fertilization? A systematic review and meta-analysis. Hum Reprod Update 2007;13: Ubaldi F, Bourgain C, Tournaye H, Smitz J, van Steirteghem A, Devroey P. Endometrial evaluation by aspiration biopsy on the day of oocyte retrieval in the embryo transfer cycles in patients with serum progesterone rise during the follicular phase. Fertil Steril 1997;67: Filicori M, Cognigni GE, Pocognoli P, Tabarelli C, Spettoli D, Taraborrelli S, et al. Modulation of folliculogenesis and steroidogenesis in women by graded menotrophin administration. Hum Reprod 2002;17: Andersen AN, Devroey P, Arce JC. Clinical outcome following stimulation with highly purified hmg or recombinant FSH in patients undergoing IVF: a randomized assessor-blind controlled trial. Hum Reprod 2006; 21: VOL. 99 NO. 1 / JANUARY

7 ORIGINAL ARTICLE: ASSISTED REPRODUCTION 33. Bosch E, Escudero E, Crespo J, Cimon C, Remohí J, Pellicer A. Serum luteinizing hormone in patients undergoing ovarian stimulation with gonadotropinreleasing hormone antagonists and recombinant follicle-stimulating hormone and its relationship with cycle outcome. Fertil Steril 2005;84: Santos MA, Kuijk EW, Macklon NS. The impact of ovarian stimulation for IVF on the developing embryo. Reproduction 2010;139: Paulson RJ, Sauer MV, Lobo RA. Embryo implantation after human in vitro fertilization: importance of endometrial receptivity. Fertil Steril 1990;53: Paulson RJ. Hormonal induction of endometrial receptivity. Fertil Steril 2011; 96: Simon C, Cano F, Valbuena D, Remohí J, Pellicer A. Clinical evidence for a detrimental effect on uterine receptivity of high serum estradiol levels in high and normal responders patients. Hum Reprod 1995;10: Groothuis PG, Dassen HH, Romano A, Punyadeera C. Estrogen and the endometrium: lessons learned fron gene expression profiling in rodents and human. Hum Reprod Update 2007;13: Lass A, Peat D, Avery S, Brinsden P. Histological evaluation of endometrium on the day of oocyte retrieval after gonadotrophin-releasing hormone agonist/follicle stimulating hormone ovulation induction for in-vitro fertilization. Hum Reprod 1998;13: Fauser BC, Devroey P. Reproductive biology and IVF: ovarian stimulation and luteal phase consequences. Trends Endocrinol Metab 2003;14: Haouzi D, Assou S, Mahmoud K, Tondeur S, Reme T, Hedon B, et al. Gene expression profile of human endometrial receptivity: comparison between natural and stimulated cycles for the same patients. Hum Reprod 2009; 24: Papanikolaou EG, Bourgain C, Kolibianakis E, Tournaye H, Devroey P. Steroid receptor expression in late follicular phase endometrium in GnRH antagonist IVF cycles is already altered, indicating initiation of early luteal phase transformation in the absence of secretory changes. Hum Reprod Update 2005;20: Labarta E, Martínez-Conejero JA, Alama P, Horcajadas JA, Pellicer A, Simon C, et al. Endometrial receptivity is affected in women with high circulating progesterone levels at the end of the follicular phase: a functional genomics analysis. Hum Reprod 2011;26: Strowitzki T, Germeyer A, Popovici R, von Wolff M. The human endometrium as a fertility-determining factor. Hum Reprod Update 2006;12: AbdelHafez FF, Desai N, Abou-Setta AM, Falcone T, Goldfarb J. Slow freezing, vitrification and ultra-rapid freezing of human embryos: a systematic review and metaanalysis. Reprod Biomed Online 2010;20: Guerif F, Bidault R, Cadoret V, Couet ML, Lansac J, Royere D. Parameters guiding selection of best embryo for transfer after cryopreservation: a reappraisal. Hum Reprod 2002;17: Mandelbaum J. Embryo and oocyte cryopreservation. Hum Reprod 2000;15: Martínez-Conejero JA, Simon C, Pellicer A, Horcajadas JA. Is ovarian stimulation detrimental to the endometrium? Reprod Biomed Online 2007;15: Horcajadas JA, Riesewijk A, Polman J, van Os R, Pellicer A, Mosselman S, et al. Effect on controlled ovarian hyperstimulation in IVF on endometrial gene expression profiles. Mol Hum Reprod 2005;11: Glujovsky D, Pesce R, Fiszbajn G, Sueldo C, Hart RJ, Ciapponi A. Endometrial preparation for women undergoing embryo transfer with frozen embryos or embryos derived from donor oocytes. Cochrane Database Syst Rev 2010;20: CD Saragusty J, Arav A. Current progress in oocyte and embryo cryopreservation by slow freezing and vitrification. Reproduction 2011;141: Prades M, Golmard JL, Schubert B, Poirot C. Embryo cryopreservation: proposal for a new indicator of efficiency. Fertil Steril 2011;95: Valojerdi M, Eftekhari-Yazdi P, Karimian L, Hassani F, Movaghar B. Vitrification versus slow freezing gives excellent survival, post warming embryo morphology and pregnancy outcomes for human cleaved embryos. J Assist Reprod Genet 2009;26: Balaban B, Urman B, Ata B, Isiklar A, Larman MG, Hamilton R, et al. A randomized controlled study of human day 3 embryo cryopreservation by slow freezing or vitrification: vitrification is associated with higher survival, metabolism and blastocyst formation. Hum Reprod 2008;23: Smith GD, Serafini PC, Fioravanti J, Yadid I, Coslovsky M, Hassun P, et al. Prospective randomized comparison of human oocyte cryopreservation with slow-rate freezing or vitrification. Fertil Steril 2010;94: Loutradi KE, Kolibianakis EM, Venetis CA, Papanikolaou EG, Pados G, Bontis I, et al. Cryopreservation of human embryos by vitrification or slow freezing: a systematic review and meta-analysis. Fertil Steril 2008;90: Cao YX, Xing Q, Li L, Cong L, Zhang ZG, Wei ZL, et al. Comparison of survival and embryonic development in human cryopreserved by slow freezing and vitrification. Fertil Steril 2009;92: Clarke JF, van Rumste MME, Farquhar CM, Johnson NP, Mol BWJ, Herbison P. Measuring outcomes in fertility trials: can we rely on clinical pregnancy rates? Fertil Steril 2010;94: VOL. 99 NO. 1 / JANUARY 2013