Culture and transfer of viable blastocysts: a feasible proposition for human IVF.

In spite of the numerous advances in the field of human assisted reproductive technologies (ART) over the past 20 years, a rate-limiting factor in the overall efficiency of the procedure (the implantation rate) has remained at 10-30%. The development of sequential media has led to the ability to culture routinely the human embryo to the viable blastocyst stage. Transfer of such blastocysts has resulted in a significant increase in implantation rates. Increases in implantation rates following blastocyst transfer have been reported for specific groups of patients culminating in the elimination of high order multiple gestations. Of greater significance, however, is that the introduction of blastocyst transfer to all patients entering infertility clinics is associated with an overall increase in implantation and pregnancy rates. Blastocysts derived from the use of sequential media are readily cryopreserved and produce high implantation rates after transfer. Using a model to account for both total embryo utilization per cycle (transferred plus cryopreserved) and implantation rate, it has been calculated that extended embryo culture and blastocyst transfer is approximately 20% more efficient than the transfer of cleavage stage embryos on day 3. Furthermore, as the score of the blastocysts obtained using sequential media is directly related to implantation and pregnancy rates, it is possible to determine which patients should be offered a single blastocyst transfer, thereby addressing the issue of twins conceived through ART.