The implementation of cryopreservation-techniques in the IVF laboratory and the improved survival rates of oocytes, cleavage and blastocyst stage embryos have led to a significant increase in the number of frozen-thawed embryo transfer cycles (FET). FETs can be planned either in a ‘pure’ natural cycle, a modified natural cycle, a stimulated cycle or a hormonal replacement therapy cycle and the optimal means to prepare the endometrium for frozen embryo transfer is a topic of ongoing controversy. Recent findings report an increased risk of hypertensive disorders if pregnancy is achieved in a frozen embryo transfer cycle without an existing corpus luteum. Therefore, the question of how to prepare the endometrium has gained even more importance and taken on a new dimension as it should not simply be reduced to the basic question of ‘which approach will result in superior pregnancy rates?’ but instead ‘which approach will result in the best pregnancy rates and the safest outcome for mother and baby?’. The aim of this review is to summarize and critically appraise the existing data on the different approaches of endometrial preparation for frozen embryo transfer with a special focus on the ‘pure’ natural cycle.

How reliable are cleavage stage and trophectoderm (TE) biopsies compared to inner cell mass (ICM) biopsies? The reliability of TE biopsy compared to ICM biopsy is almost perfect, but only substantial between cleavage stage biopsy and ICM biopsy. One of the prevailing reasons for implantation failure is presumed to be chromosomal aneuploidy in human preimplantation embryos. Preimplantation genetic testing for aneuploidies (PGT-A) has been introduced into assisted reproduction in an effort to increase pregnancy rates. Increasing evidence indicates that genetic results obtained following blastomere or TEbiopsy may not accurately reflect the true genetic status of the embryo due to the presence of embryonic mosaicism, and therefore the reliability of PGT is highly controversial. This was an observational descriptive study, performed in a private infertility centre from August 2016 to January 2017. The mean female age was 33.9 years, ranging from 24 to 46 years, and the mean number of biopsied embryos per couple was 2.2 (range 1-7 embryos). Blastomere biopsies had been performed at cleavage stage on Day 3 (D3) due to the turnover time of genetic testing and the inability to cryopreserve embryos in accordance with the local law governing ART. To confirm the genetic results in embryos not chosen for transfer, additional biopsies of the TE at blastocyst stage (BLASTO-TE) as well as of the ICM (BLASTO-ICM) were performed on D5. Only surplus blastocysts, which had not been selected for transfer and were not cryopreserved in accordance with the law governing ART, had been included. Comparison of all biopsies (D3/BLASTO-ICM/BLASTO-TE) per embryo demonstrated that 50 (59.5%) out of 84 embryos showed concordance in all three results (= full concordance). Thirty-four (40.4%) embryos had at least two discordant results between the three biopsies, regardless of whether the embryo diagnosis (aneuploid/euploid) was discordant or not, or in aneuploid embryos, whether the chromosomal patterns were inconsistent. Nine (= 10.7%) embryos had complete discordance between all three biopsies. False positive results between D3/BLASTO-TE, D3/BLASTO-ICM and BLASTO-TE/BLASTO-ICM were 26.4%/30.2% and 7.5%, respectively, while the Kappa agreement between the different approaches was 0.647, 0.553 and 0.857, respectively. Therefore the reliability of D3/BLASTO-TE, D3/BLASTO-ICM and BLASTO-TE/BLASTO-ICM can be interpreted as substantial, as moderate and as almost perfect. The limitation of this study is the possible bias in the concordance/discordance rate because embryos that had been selected for transfer did not undergo biopsy on D5. The obvious discordance between the three different approaches for PGT-A underlines the limitations of genetic testing and highlights the importance of ongoing research in order to improve the accuracy of PGT-A results. Until then reproductive specialists will continue to make challenging decisions on whether to transfer or discard an embryo in light of current evidence questioning the reliability of genetic results. This study was supported by Igenomix. The funder provided support in the form of salary for R.C. The co-author R.C. is an employee of Igenomix. She participated in the blinded analysis of the samples; however the final data collection and statistical analysis of the results, as well as the decision to publish, was taken by B.L, I.E. and H.F. The authors B.L., I.E., A.L., A.B., A.A., N.D. and H.F. have no competing interests. The funder did not have any additional role in the study design, data collection and analysis, decision to publish or preparation of the manuscript. The commercial affiliation of R.C. did not play any role in the study. This study was approved by the Ethics Committee of IVIRMA Middle East Fertility Clinic, Abu Dhabi, UAE (Research Ethics Committee IVI-MEREFA009a/2017).

STUDY QUESTION Does hormonal stimulation with corifollitropin alpha (CFA) only, mimicking a step down protocol, result in lower incidence of progesterone elevation on the day of hCGtrigger as compared to sustained stimulation with recombinant FSH (rFSH)? SUMMARY ANSWER The current findings support the concept that sustained FSH stimulus contributes to premature progesterone elevation in stimulated IVF cycles. WHAT IS KNOWN ALREADY Serum progesterone rise during the follicular phase of ovarian stimulation for IVF treatment seems to be related to a poorer reproductive outcome. However, the mechanism by which the rise in progesterone is caused is not yet fully understood. STUDY DESIGN, SIZE, DURATION This study was a post hoc analysis of data from two multi-center, randomized, double-blind, double-dummy, active-controlled, non-inferiority trials, ENGAGE and PURSUE, conducted from June 2006 to January 2008 and from July 2010 to October 2012 respectively. PARTICIPANTS/MATERIALS, SETTING, METHODS In the ENGAGE-study, 1506 women, aged 18–36 years, were allocated to either a single injection of 150 mg CFA or daily injections of 200 IU rFSH in the first week of stimulation, using a standard GnRH antagonist protocol. In the PURSUE-study, a total of 1390 women, aged 35–42 years, were allocated to either a single injection of 150 mg of CFA or daily 300 IU of rFSH for the first week, again using a standard GnRH antagonist protocol. In both trials, daily rFSH was continued until three follicles reached >17 mm in size. All women had a body weight of between 50 and 90 kg, regular menstrual cycles and an indication for ovarian stimulation before IVF. The incidence of progesterone elevation on day of hCG-trigger in patients with CFA only or rFSH stimulation, and triggered on Day 8 of stimulation, was analyzed. MAIN RESULTS AND THE ROLE OF CHANCE Of patients with CFA only stimulation, 5.4% (13/239 patients) showed a progesterone elevation above 1.5 ng/ml on day of hCG-trigger, whereas patients with rFSH stimulation had a significant higher incidence of progesterone elevation (18.3%; 62/339 patients) (P < 0.001). LIMITATIONS, REASONS FOR CAUTION Post hoc analysis of data from previously published trials could be considered as a reason for caution. WIDER IMPLICATIONS OF THE FINDINGS Future studies should evaluate whether it would be possible to prevent a premature progesterone rise in cycles stimulated with daily FSH by using a step down protocol towards the end of the follicular phase. STUDY FUNDING/COMPETING INTEREST(S) Financial/Material Support was provided by Merck & Co., Inc., Kenilworth, NJ, USA. Davis Gates is an employee of Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA and may own stock and/or hold stock options in the company. Fabiola Beligotti is an employee of MSD, Italy, a subsidiary of Merck & Co., Inc., Kenilworth, NJ, USA and may own stock and/or hold stock options in the company. Barbara Lawrenz, Nils Engelmann and Human M. Fatemi have no conflict of interest. TRIAL REGISTRATION NUMBER ENGAGE study: ClinicalTrials.gov identifier NTC00696800. PURSUE-study: NCT01144416.