What is a blastocyst? What advantages are there to transferring the embryo on day 5?

A blastocyst is an embryo which has been left to develop until day 5 or 6 and presents a complex cellular structure formed by approximately 200 cells. The blastocyst phase is the development stage prior to implantation of the embryo in the mother’s uterus.

Transferring a blastocyst on day 5+ has the following advantages:


  • It makes a better selection of embryos possible. Embryos which reach this stage offer a better implantation capacity due to the fact that they have overcome possible development hurdles which are frequently produced in the early stages. It is for that reason that we limit transfers to 1 blastocyst (or maximum 2), reducing the risk of multiple births.
  • It allows a more physiological synchronisation between the embryonic phase and the responsiveness of the endometrium.
  • Current Spanish Law allows for day 5 transfers, which facilitates the correct supervision of embryo development and the transfer of only the embryos selected by Nature to reach an optimum state of development, which in turn allows for a higher implantation rate.

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