When early cleavage stage transfers embryos remain in the female tract they become exposed to gonatropins for longer periods of time than necessary. Thus, early embryo transfers are usually pushed back to day 3 instead of day 2. This reason for the shift to day 3 is due to the observation that a large percentage of embryos arrest between the 4-8 cell stage in vivo. As you can see early cleavage stage transfers are a viable option but day 3 seemed to be a the best balance between the need to avoid embryo arrest with single media and the need to reduce exposure to the female tract after gonadtropin administration.
Another disadvantage associated with early transfers, is the need to transfer a high number of embryos. With early transfers it is more common to transfer more embryos since the quality of the embryo is more difficult to see at this early stage. In addition, there have been some differing opinions about the scoring criteria for early embryo transfers (7). Pantos believes that the scoring criteria cannot accurately predict pregnancy potential since the scoring criteria is not accurate. Embryos transferred at this early stage produce low implantation rate, between 10-20% (5).
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