The routine culture and transfer of viable human blastocysts has been made possible by the development of sequential culture media, formulated to account for the changes in nutrient requirements of the embryo as it develops and differentiates. Resultant implantation rates of blastocysts transferred on day 5 are significantly higher than those obtained by the transfer of cleavage stage embryos transferred on day 2 or day 3 within the same programme. As a direct result of this increase in implantation rate, fewer blastocysts than cleavage stage embryos need to be transferred to obtain acceptable pregnancy rates, thereby reducing the incidence of multiple gestations. Blastocysts developed in sequential culture media are readily cryopreserved. The efficiency of in vitro fertilization (IVF) in a general patient population can be calculated using a model that takes into account the number of embryos transferred and cryopreserved, together with their respective implantation rates. Blastocyst transfer is associated with about a 20% increase in the efficiency of IVF compared with the transfer of cleavage stage embryos on day 3. The development of a suitable scoring system has enabled identification of those blastocysts with the highest developmental potential (70% implantation rate). The culmination of this work should be the move to the transfer of a single blastocyst for a significant number of patients.
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