Zygote 21 (February), pp. 77–83. C  Cambridge University Press 2011 doi:10.1017/S096719941100058X First Published Online 6 October 2011 Advantages of the two-step embryo transfer strategy in human IVF/ICSI cycles Chadi Yazbeck 1,3 , Nadia Ben Jamaa 2 , André Hazout 2,4 , Paul Cohen-Bacrie 4 , Anne-Marie Junca 4 and Nathalie Rougier 2 Department of Obstetrics, Gynaecology and Reproductive Medicine, IVF unit, Bichat Claude Bernard University Hospital, Paris; INSERM U1018 – CESP, Villejuif; and Department of Reproductive Biology, Eylau – La Muette IVF Clinic, Paris, France Date submitted: 01.11.10. Date accepted: 14.05.11 Summary The aim of this study was to evaluate the advantages of the two-step embryo transfer (ET) strategy combining a day 2/3 ET with a day 5/6 blastocyst transfer. In an observational comparative study, 400 infertile women were enrolled from two assisted reproductive technology (ART) units according to inclusion criteria: age below 42 years and at least three embryos obtained on day 2 thus allowing an extended in vitro culture. Two groups were defined according to the ET strategy adopted: group 1 had a two-step ET; and group 2 had a day 2/3 ET with (subgroup 2a) or without (subgroup 2b) blastocysts cryopreserved on day 5/6. Live birth rate was significantly higher in group 1 than in subgroups 2a and 2b (36.5% versus 29.4% and 13.4%, respectively; p < 10 −3 ). Multiple pregnancy rates were comparable between groups. After adjusting on major prognostic factors, the two-step ET strategy was still associated with a significantly higher live birth rate than the day 2/3 ET (OR = 2.23; 95% CI: 1.32–3.77). The two-step ET provides better live birth rates than the cleavage-stage ET. It does not increase multiple pregnancy rates if the number of embryos transferred is limited. It also prevents cycle loss when embryos fail to develop into blastocysts. Keywords: ART, blastocyst, extended culture, prognostic factors, two-step embryo transfer Introduction During the last decade, the popularity of extended embryo culture up to day 5 has increased in in vitro fertilization (IVF) practice (Wilson et al., 2004). Advocates of blastocyst culture believe that only the most viable embryos will survive the extended culture until day 5/6, which results in a higher probability of implantation (Ashkenazi et al., 2000). This theory also 1 All correspondence to: Chadi Yazbeck. Service de Gynécologie Obstétrique, Hôpital Bichat Claude Bernard, 46 rue Henri Huchard, 75018 Paris, France. Tel: +33 1 40 25 77 20. Fax: +33 1 40 25 76 00. e-mail: chayazmd@hotmail.com 2 Department of Obstetrics, Gynaecology and Reproductive Medicine, IVF unit, Bichat Claude Bernard University Hospital, AP-HP, 75018 Paris, France. 3 INSERM U1018 – CESP, 94807 Villejuif, France. 4 Department of Reproductive Biology, Eylau – La Muette IVF Clinic, 75116 Paris, France. leans on the fact that, at the day 2/3 stage, it is difficult to predict accurately which embryo will be capable of further development (Nyboe Andersen et al., 2004). This shift in IVF practice was also motivated by a better correlation between blastocyst morphology and euploidy, a better synchronization between the embryo and the endometrium (Goto et al., 2005; Papanikolaou et al., 2006) and a lower contractility of the uterus at that time (Fanchin et al., 2001). Nevertheless, a day 5/6 embryo transfer (ET) strategy has not been convincing because of certain drawbacks such as the risk of cycle cancellation when no embryos develop into blastocysts in vitro, the decreased number of frozen embryos and their poor survival after thawing, which is a side effect of the slow freezing procedure (Guerif et al., 2009). In fact, a recent meta-analysis confirmed that higher clinical pregnancy and live birth rates were associated with day 5/6 ET when compared with day 2/3 ET. These results were obtained at the price of higher cancellation and lower cryopreservation rates when