The transfer of early cleavage embryos in the 4-cell or 8-cell stage has been extensively evaluated in numerous studies, and early cleavage transfers remain the most widespread stage of transfer to date. Early cleavage transfers became popular due to the fact that the culture media initially used was not sufficient to support embryo development to day 4 or blastocyst stage.
With a lack of appropriate culture media, embryos cultured past the 4-8 cell stage were not surviving. Although embryos transferred at day 2 or 3 are placed back into the uterus prematurely compared to what occurs naturally, without the proper culture media, the uterus is the optimal place for embryo survival.
Early cleavage stage transfers have advantages that aid in their popularity throughout laboratories. Cleavage stage transfers have been used regularly in IVF for two main reasons: First, embryos cultured past cleavage stage have a low developmental rate (3) Secondly, in contrast to other primates, human embryos have the unusual ability to survive when replaced prematurely into the uterus (4). The uterus is believed to provide the best environment for an embryo’s survival. In vivo an embryo enters the uterus in the morula stage, 4-5 days after ovulation. Humans have the ability to support early cleavage stage transfers, but the embryos resided in the fallopian tubes until day 4 (6).
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