Cycles with progesterone elevation during controlled ovarian stimulation (COS) for IVF/ICSI are commonly managed with a “freeze-all” strategy, due to a well-recognized detrimental effect of high progesterone levels on endometrial receptivity. However, also a detrimental effect of elevated progesterone on day-3 embryo quality has recently been found with regards to top quality embryo formation rate. Because blastocyst culture and cryopreservation are largely adopted, we deemed relevant to determine whether this detrimental effect is also seen on blastocyst quality on day 5–6. This issue was investigated through a large two-center retrospective study including 986 GnRH antagonist IVF/ICSI cycles and using top quality blastocyst formation rate as the main outcome. Results showed that on multivariate analysis sperm motility (p1.49 ng/ml as the best cut-off for identification of patients at risk for the absence of top quality blastocysts (AUC 0.55, p<0.01). Our study is the first to investigate the top quality blastocyst formation rate in relation to progesterone levels in IVF/ICSI cycles, showing that increasing progesterone is associated with lower rates of top quality blastocyst. Hence, the advantages of prolonging COS to maximize the number of collected oocytes might eventually be hindered by a decrease in top quality blastocysts available for transfer, if increasing progesterone levels are observed. This observation extends the results of two recent studies focused on day-3 embryos and deserves further research.

STUDY QUESTION Are women with endometriosis who conceive with IVF at increased risk of preterm birth? SUMMARY ANSWER Women with endometriosis who conceive with IVF do not face an increased risk of preterm birth. WHAT IS KNOWN ALREADY The eutopic endometrium of women with endometriosis has been repeatedly shown to present molecular and cellular alterations. On this basis, it has been hypothesized that pregnancy outcome may be altered in affected women. However, to date, available evidence from epidemiological studies is scanty and conflicting. Data tended to be partly consistent only for an increased risk of preterm birth and placenta previa. STUDY DESIGN, SIZE, DURATION Retrospective matched case–control study of women achieving an IVF singleton pregnancy progressing beyond 12 weeks’ gestation. PARTICIPANTS/MATERIALS, SETTING, METHODS Women achieving IVF singleton pregnancies that progressed beyond 12 weeks' gestation at two infertility units were reviewed. Cases were women with a history of surgery for endometriosis and/or with a sonographic diagnosis of the disease at the time of the IVF cycle. Controls were women without current or past evidence of endometriosis who were matched to cases by age (± 6 months), type of cycle (fresh or frozen cycle) and study period. Male factor and unexplained infertility were the most common diagnoses in the control group. Two hundred and thirty-nine women with endometriosis and 239 controls were selected. The main outcome of the study was the rate of preterm birth (birth < 37 weeks' gestation) regardless of the cause. Secondary analyses were performed for the most common obstetrical complications. MAIN RESULTS AND THE ROLE OF CHANCE The rate of preterm birth was similar in the two study groups (14% and 14%, respectively, p = 0.89). The rate of live birth and the incidence of hypertensive disorders, gestational diabetes, small and large for gestational age newborns and neonatal problems also did not differ. In contrast, placenta previa was more common in women with endometriosis than controls (6% versus 1%, respectively; p = 0.006): The adjusted odds ratio was 4.8 (95% confidence interval: 1.4–17.2). LIMITATIONS, REASONS FOR CAUTION As for all observational studies, confounders cannot be totally excluded. Moreover, the retrospective study design exposes the findings to some inaccuracies. For example, the independent role of adenomyosis could not be reliably assessed because this diagnosis is complex and would necessitate a prospective recruitment. Second, the selection of controls may also be a matter of concern because some affected women may have been erroneously included in this group. Third, even if the sample size is significant, it is insufficient for robust subgroup analyses. Finally, it is mandatory to point out that our conclusions are valid for IVF pregnancies only, and specific data from properly designed studies are required to support any inference for natural pregnancies. WIDER IMPLICATIONS OF THE FINDINGS The results of our study suggest that women with endometriosis conceiving with IVF can be reassured regarding the risk of preterm birth. The observed association with placenta previa requires further investigation and may open a new avenue of research. STUDY FUNDING/COMPETING INTEREST(s) No external funding was used for this study. None of the authors have any conflict of interest to declare.

There is growing interest on the potential clinical relevance of the endometrial microbiome. However, insufficient attention has been given to the methodology of sampling. To minimize contamination, we advocate the use of the double-lumen catheters commonly employed for the embryo transfer. Endometrial fluid samples obtained from 53 women scheduled for IVF were studied for microbiome characterization. Control samples from the vagina of these same women were concomitantly obtained. Samples were analysed by V3–V4–V6 regions of 16S rRNA gene sequencing with Next Generation Sequencing technique. Endometrial Lactobacillus-dominant cases were uncommon compared to previous evidence, being observed in only 4 (8%) women. Taxonomy markedly differed between the endometrial and vaginal microbiomes composition. The most common bacterial genera coincided in only 4 (8%) women. The comparison between women who did and did not subsequently become pregnant failed to identify any microorganism associated with the success of the procedure. However, the endometrial biodiversity resulted higher among pregnant women. Shannon’s Equitability index in pregnant and non pregnant women was 0.76 [0.57–0.87] and 0.55 [0.51–0.64], respectively (p = 0.002). In conclusion, the use of embryo transfer catheters for testing the endometrial microbiome is promising. The scant concordance with vaginal samples supports the validity of this approach. Moreover, our study highlighted a possible beneficial role of a higher biodiversity on endometrial receptivity.