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Embryos are normally transferred 2 to 3 days after the egg collection as it is thought to be the most appropriate stage to replace the embryos back into the uterus. It also allows us to offer embryo freezing and assisted hatching at the most successful stages. However in natural cycles embryos would not normally reach the uterus until day 5, at the blastocyst stage of development.
Recently, however, new types of media have become available that give the embryos the necessary changes in nutrition so that we can keep the embryos in culture in the laboratory until around day 5. However around 50% of human embryos stop developing before the blastocyst stage. The advantage of letting the embryos grow in culture for longer is that we can select embryos that continue to develop into blastocysts which may help in selecting the embryos with better potential. The fact that an embryo has greater potential to grow than the others does not necessarily mean that a pregnancy will definitely occur as other factors are also important, such as the endometrial lining of the uterus, the age of the patient and the outcome of previous cycles.
Download further info here (* in pdf format)
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