Abstract: The supra-physiologic hormone levels observed during controlled ovarian stimulation (COS) may lead to morphologic and biochemical endometrial alterations. These changes may jeopardize embryo implantation during fresh embryo transfer cycles. With improvements in the cryopreservation techniques, the quality and potential of implantation of cryopreserved embryos remains similar to those from fresh cycles. Thus, although the fresh embryo transfer is the norm during in vitro fertilization (IVF) cycles, there is increasing interest in the elective frozen-thawed embryo transfer (freeze-all policy). The main objective of this study was to compare IVF outcomes between fresh embryo transfer and freeze-all policy in normal responders Prospective observational cohort study The study was conducted between January 2012 and December 2015. A total of 938 IVF cycles (fresh group - 523; freeze-all group - 415) submitted to COS with gonadotropin-releasing hormone (GnRH) antagonist protocol and cleavage stage embryo transfer were included. In the fresh group embryo transfers were performed only if progesterone levels were <= 1.5 ng/mL on the trigger day. The analysis was also performed in three sub-groups of patients based on the number of retrieved oocytes: group 1 (12-15 oocytes), group 2 (8-11 oocytes), and group 3 (4-7 oocytes). The main outcome measure was ongoing pregnancy rate. The secondary outcomes were implantation, pregnancy, and clinical pregnancy rates In general, there were a higher implantation (25.5% versus 19.3%, p=0.002) and ongoing pregnancy rate (41% versus 32%, p=0.004) in freeze-all group when compared to fresh group. The sub-group analysis is presented in Table 1. There was no statistical difference between fresh and freeze-all groups in ovarian reserve tests, days of stimulation, total dose of gonadotropin, and fertilization rates. This is the largest study evaluating the freeze-all strategy and the first to evaluate sub-groups of normal responders based on ovarian response to ovarian stimulation. The results of this study show that although the freeze-all policy may be related to better IVF outcomes in normal responders, these potential advantages decrease with worsening ovarian response. Patients with poorer ovarian response don't have benefit from the freeze-all strategy.Tabled 1Table 1. Fresh vs. elective FET in normal responders - sub-group analysisGroup 1(12-15 oocytes)Group 2(8-11 oocytes)Group 3(4-7 oocytes)FreshFreeze-all (p)FreshFreeze-all (p)FreshFreeze-all (p)No of patients69109172140282166Age, years +/- SD33.20 +/- 5.1633.81 +/- 3.70 (p=0.400)34.98 +/- 4.7434.32 +/- 4.72 (p=0.221)36.55 +/- 4.3535.78 +/- 4.15 (p=0.066)Retrieved oocytes +/- SD13.26 +/- 1.1313.44 +/- 1.13 (p=0.305)9.33 +/- 1.129.29 +/- 1.13 (p=0.798)5.51 +/- 1.085.48 +/- 1.07 (p=0.811)No of embryos transferred +/- SD2.28 +/- 0.682.25 +/- 0.56 (p=0.770)2.28 +/- 0.732.18 +/- 0.71 (p=0.099)2.34 +/- 0.812.22 +/- 0.60 (p=0.067)Implantation Rate, %16.6%28.6% (p=0.039)19.6%26.5% (p=0.038)17.4%18.2% (p=0.490)Pregnancy Rate, %39.1%56.9% (p=0.021)43.0%55.0% (p=0.035)39.4%41.6% (p=0.647)ClinicalPregnancy Rate, %33.3%53.2% (p=0.009)40.1%51.4% (p=0.047)34.7%36.7% (p=0.671)Ongoing Pregnancy Rate, %31.9%47.7% (p=0.035)34.9%47.1% (p=0.029)30.1%31.3% (p=0.793)Relative Risk Ongoing Pregnancy-Freeze-all vs Fresh1.50 (1.01-2.23)1.35 (1.03-1.77)1.04 (0.78-1.38) Open table in a new tab