The embryo In Vivo normally spends the following days, after fertilization, going down the Fallopian tubes and multiplying quickly its number of cells through a division, and in this way, the embryo starts its development and differentiation. The Blastocyst is a much differentiated structure formed by many cells and an external membrane. The Blastocyst continues its way down the Fallopian tubes and 7-9 days after fertilization, it will be able to – if everything has been successful - implant in the uterus.
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Blastocyst "hatching" from its zona pellucida |
The development to the stage of Blastocyst is one of the critical points in the embryo growth and differentiation; therefore we sometimes opt to wait, in order to select the best embryos, until this stage by keeping the embryos in an In Vitro culture for a longer period than we frequently do. In general, embryo transfer takes place 48-72 hours after oocyte pick-up, this means on the day 2-3 of the embryo development. When Blastocyst embryo transfer is performed, the embryo is keep in culture until day 5-6 after oocyte pick up.
In order to do so, we use commercial sequential culture media to allow the embryo to develop up to these more advanced stages. They are kept into the incubator until day 5-6 when they are once more evaluated in terms of development.