Introduction, In recent years, much progress has been made in improving embryo quality and selection for transfer after in vitro fertilization (IVF). These include technical improvements in oocyte and embryo culture conditions, assisted hatching, the use of sequential media, and the use of embryo-endometrial coculture for selected patients. Moreover, preimplantation genetic screening (PGS) has been introduced with the aim of improving embryo selection on the basis of chromosomal compliment. However, despite these advances, even when embryos are considered to be of high quality using morphological and chromosomal criteria, implantation rates remain around 25–35 percent per embryo transfer procedure (The European IVF-monitoring program, 2005). High rates of implantation failure and early pregnancy loss in IVF continue to be a considerable cause of frustration to both patients and clinicians. The practice of transferring multiple embryos to increase the chance of implantation continues in contemporary practice. However, this approach is now being seen less as the “solution” to low implantation rates but rather more as the cause of the most important problem arising from IVF treatment: that of multiple pregnancy (Fauser et al., 2005). The widespread adoption of single-embryo transfer (SET) is recognized to be necessary if the morbidity associated with IVF multiple pregnancies is to be reduced (Fauser et al., 2005). In order to improve implantation rates in the age of SET, medical strategies designed to increase the chance of successful implantation are sought and applied in practice, often before they have been subject to appropriate clinical assessment in the context of randomized controlled trials (RCTs) (Boomsma and Macklon, 2006).