STUDY QUESTION Is there a difference in clinical pregnancy and live birth rates (LBRs) between blastocysts developing on Day 5 (D5) and blastocysts developing on Day 6 (D6) following fresh and frozen transfers? SUMMARY ANSWER D5 blastocyst transfers (BTs) present higher clinical pregnancy and LBRs than D6 in both fresh and frozen transfers. WHAT IS KNOWN ALREADY BT is increasingly popular in assisted reproductive technology (ART) centers today. To our knowledge, no meta-analysis has focused on clinical outcomes in both fresh and frozen BT. Concerning frozen blastocysts, one meta-analysis in 2010 found no significant difference in pregnancy outcomes between D5 and D6 BT. Since then, ART practices have evolved particularly with the wide use of vitrification, and more articles comparing D5 and D6 BT cycles have been published and described conflicting results. STUDY DESIGN, SIZE, DURATION Systematic review and meta-analysis of published controlled studies. Searches were conducted from 2005 to February 2018 on MEDLINE and Cochrane Library and from 2005 to May 2017 on EMBASE, Eudract and clinicaltrials.gov, using the following search terms: blastocyst, Day 5, Day 6, pregnancy, implantation, live birth and embryo transfer (ET). PARTICIPANTS/MATERIALS, SETTING, METHODS A total of 47 full-text articles were preselected from 808 references, based on title and abstract and assessed utilizing the Newcastle–Ottowa Quality Assessment Scales. Study selection and data extraction were carried out by two independent reviewers according to Cochrane methods. Random-effect meta-analysis was performed on all data (overall analysis) followed by subgroup analysis (fresh, vitrified/warmed, slow frozen/thawed). MAIN RESULTS AND THE ROLE OF CHANCE Data from 29 relevant articles were extracted and integrated in the meta-analysis. Meta-analysis of the 23 studies that reported clinical pregnancy rate (CPR) as an outcome, including overall fresh and/or frozen ET cycles, showed a significantly higher CPR following D5 ET compared with D6 ET (risk ratio (RR) = 1.27, 95% CI: 1.15–1.39, P < 0.001). For CPR, calculated subgroup RRs were 2.38 (95% CI: 1.74–3.24, P < 0.001) for fresh BT; 1.27 (95% CI: 1.16–1.39, P < 0.001) for vitrified/warmed BT; and 1.15 (95% CI: 0.93–1.41, P = 0.20) for slow frozen/thawed BT. LBR was also significantly higher after D5 BT (overall RR = 1.50 (95% CI: 1.32–1.69), P < 0.001). The LBR calculated RRs for subgroups were 1.74 (95% CI: 1.37–2.20, P < 0.001) for fresh BT; 1.38 (95% CI: 1.23–1.56, P < 0.001) for vitrified/warmed BT; and 1.44 (95% CI: 0.70–2.96, P = 0.32) for slow frozen/thawed BT. Sensitivity analysis led to similar results and conclusions: CPR and LBR were significantly higher following D5 compared to D6 BT. LIMITATIONS, REASONS FOR CAUTION The validity of meta-analysis results depends mainly on the quality and the number of the published studies available. Indeed, this meta-analysis included no randomized controlled trial (RCT). Slow frozen/thawed subgroups showed substantial heterogeneity. WIDER IMPLICATIONS OF THE FINDINGS In regards to the results of this original meta-analysis, ART practitioners should preferably transfer D5 rather than D6 blastocysts in both fresh and frozen cycles. Further RCTs are needed to address the question of whether D6 embryos should be transferred in a fresh or a frozen cycle. STUDY FUNDING/COMPETING INTEREST(S) This work was sponsored by an unrestricted grant from GEDEON RICHTER France. The authors have no competing interests to declare. REGISTRATION NUMBER CRD42018080151.

This study had sufficient power to detect doubling of the PIH rate. It was demonstrated that the risk of PIH was tripled for ED versus controls. Even in young women, ED is a risk factor for PIH. An immunological explanation seems most likely, that is, the fetus is fully allogeneic to its mother. This risk must be acknowledged to inform couples and provide careful pregnancy monitoring.

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