Blastocyst Transfer: Means of Overcoming Disadvantages Focused on Embryo Selection

Cleavage-stage embryos have been transferred to the uterus since the first successful in vitro fertilization-embryo transfer (IVF-ET). Physiologically the embryos transferred to the uterus should be at the blastocyst stage, but cultivation to the blastocyst stage was difficult because of a poor culture environment. At the first attempt reported by Bolton et al. in 1989 [1] and 1991 [2] only 17% of embryos reached the expanded blastocyst stage and the implantation rate for blastocyst transfer (BT) was 7%. Subsequently, Menezo et al. [3] introduced a coculture system with Vero cells, with which 55(cid:150)60% of embryos reached the blastocyst stage. With the sequential medium, it is now possible to culture embryos to the blastocyst stage without feeder cells [4]. Gardner et al. [5] reported that the proportion of embryos reaching the blastocyst stage was 46.5% and the implantation rate for BT was 50.5%. Furthermore, their comparison study with conventional embryo transfer at the cleavage stage (ET) showed that the implantation rate for BT was much higher than that for ET [5].