CONTEXT: As the control ovarian hyperstimulation(COH) has negative impact on the uterine environment, that may affect the implantation process or the development of the pregnancy, a “freeze-all” strategy has been taken into consideration, with cryopreservation of all embryos and subsequent transfer. OBJECTIVE:To evaluate the effectiveness and the obstetric and perinatal outcome of the freeze-all strategy (frozen-thawed ET) (FET) compared to the fresh embryo transfer(ET). METHODS:We performed a systematic review of studies available in Cochrane Central Register of Studies, MEDLINE, CINAHL. RESULTS: A retrospective study on 853 patients found that the live birth rates(LBR) were significantly higher in the “freeze-all” group than those in the control group(64.3% vs. 45.8%), while a meta-analyse of 4 randomised clinical trials including 1892 women concluded that there was no statistic significant difference in LBR, but they reported a significantly decrese of the OHSS in the FET group, a lower rate of miscarriages but a higher rate of pregnancy complications. Another study that included 3 trials with 633 cycles showed that FET resulted in significantly higher PR (RR 1.32), while fresh ET had higher miscarriage rate, but not statistical significant enough (RR 0.83).A Catalan cohort study on 14262 newborns showed that after fresh ET there is a higher risk of SGA and concluded that COH negatively affects the perinatal outcome while this is not affected by the vitrification process. A meta-analysis of 11 observational studies concluded that singleton pregnancies resulting from FET are associated with lower obstetric risk and perinatal morbidity(antepartum hemorrhage, preterm birth (PB), SGA, low birth weight (LBW), and perinatal mortality 0.67, 0.84, 0.45, 0.69, and 0.68). A Nordic study confirms many of these findings: reduced LGW, PB, and SGA for the FET ([95% CI] = 0.81, 0.84, 0.72), but a higher risk of LGA and perinatal mortality (aOR 1.49).Another study on 1166 blastocyst transfers concluded that FET resulted in higher implantation rates(51.5% vs.40.6%), LBR (56.8% vs.44.3%) and lower SGA and ectopic pregnancy(0.32% vs. 1.80%), while miscarriage, premature delivery, perinatal morbidity were nonsignificantly different. CONCLUSIONS:Clinically significant differences suggest better endometrial receptivity and placentation in FET cycles, consequent higher BR, and lower risk of SGA, PB and perinatal morbidity, making the “freeze-all” policy an eligible protocol.