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Blastocysts are embryos that were able to develop in the incubator for 5 days after follicle puncture and fertilization and consist of 64 to 128 cells.

Of all fertilised eggs, only about one third develop into a blastocyst at all under culture conditions. In a blastocyst transfer, the embryos are transferred into the uterus on the 4th or 5th day after egg collection. Due to the longer culture, the transfer takes place at the time when the embryos usually reach the uterus and settle down. Blastocyst culture is permitted in Germany - the transfer of the fertilised eggs can take place between day 1 and day 5 after egg collection. 24 hours after egg collection, the eggs in fertilisation (PN stages) are selected to remain in the cell culture. In animal experiments, indications of a slightly increased risk of malformations were observed in extended cultures after day 3.