As part of the process of In-Vitro Fertilization (IVF), embryos are usually transferred into the woman's uterus on day 3 following fertilization. At day 3 of development, the embryos consist of approximately 4 to 8 cells. It is very difficult at this early stage to determine which embryos are most likely to continue developing to the next stage, the blastocyst stage, and will ultimately be capable of hatching from the Zona Pellucida (the outermost membrane surrounding the embryo). While many embryos appear to be developing normally under the microscope by 3 days following fertilization, some of them may arrest in development and are not able to develop further. As a result of limitations in modern science, it is not always possible to distinguish between embryos which have a higher probability of reaching the blastocyst stage and embryos that will arrest in development and hence will not develop further. In an attempt to resolve this problem, a new IVF technique has been developed that is known as Prolonged In-Vitro Culture to the Blastocyst Stage. This technique involves allowing the embryos to continue to develop in the laboratory until they reach the blastocyst stage, at which time they are transferred into the woman's uterus or cryopreserved. Prolonged In-Vitro Culture may have advantages over traditional IVF techniques. Firstly, transferring blastocysts may allow for better synchronization between the stage of development of the embryos and the stage of development of the endometrum (the lining of the uterus). The 4 to 8 cell embryos that are transferred using traditional IVF techniques would, if fertilization occurred naturally, normally be found in a woman's fallopian tubes (the tubes from the ovaries to the uterus) and not in the uterus, as the developing embryo would not have reached the uterus by that stage. It is felt that this better synchronization between the embryos and the endometrial lining may improve the likelihood of implantation. Secondly, by transferring fewer embryos that are more developmentally advanced, the risk of multiple pregnancies may be reduced.

As with traditional IVF techniques and Intra-Cytoplasmic Sperm Injection, Prolonged In-Vitro Culture may result in a surplus of embryos, in this case blastocysts, which may be cryopreserved (frozen) for transfer into the uterus in future treatment cycles.

The main risk of Prolonged In-Vitro Culture to the Blastocyst Stage is that none of the fertilized embryos may actually develop to the blastocyst stage, and thus no embryos will be available for transfer. It is not certain that all embryos that fail to develop in vitro, would have failed to develop in the womb.

The information presently available from the medical literature indicates that Prolonged In-Vitro Culture to the Blastocyst Stage is not associated with any increase in congenital, physical or genetic abnormalities in children following its use, in excess of the normal and expected maternal age-matched incidence in children born of naturally conceived pregnancies.