Another advantage of early replacement of embryos into the uterus is to limit the time a embryo/s spends outside a in vivo environment in a embryology laboratory. The longer the embryos stay outside its natural environment the higher the chance of contamination, complications, and potential arrest of embryos. Contamination can occur either by an error by the embryologists or in faulty laboratories equipment and supplies. With the lack of appropriate culture media and the ability to transfer embryos back prematurely to the safe uterine environment, early cleavage stage transfers made the most sense. Therefore, it has been a common practice to transfer embryos to the uterus on day 2 or 3 when the embryos are in the 4-8-cell stage (5). (taguchi, saguri,PAB1)
With the introduction of stage specific media and blastocyst transfers, the imperfections of early cleavage transfer have become more apparent. The environment of the female tract after gonadtropin administration is not as supportive to the embryo as a non-stimulated environment. Taking into consideration this delicate process it seems that transferring embryos after the administration of gonadortopins can be harmful. A embryo is better suited to only be expose to this type of environment for the shortest period of time possible; which would mean a later transfer. With a later transfer, the embryo would not have to sit in the fallopian tubes.
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