Early Cleavage stage transfers have been used regularly in IVF for two main reasons: First, embryos cultured past cleavage stage have a low developmental rate. Secondly, in contrast to other primates, human embryos have the unusual ability to survive when replaced prematurely into the uterus. Although this technique provides many benefits, cleavage stage embryo transfers lack the vital ability to select the best quality embryos for transfer to increase patient reproductive success.
Transfer of embryos at the blastocyst stage provides the ability for a laboratory to have such selection of higher quality embryos. At day 2 or 3, the embryos have not been given enough chance to grow and develop for an embryologist to select the best quality embryos. A large proportion of embryos that are morphologically normal on day 3 are actually chromosomally abnormal. This is part of the reason why there is an 80-90% implantation rate failure seen in cleavage stage embryo transfers. Although embryo transfers on day 5 cannot guarantee the absence of chromosomally abnormal embryos, they can in fact reduce the chance of this chromosomal abnormality. Since early cleavage stage have a low implantation rate and inaccurate scoring criteria, it is common for clinics to transfer a greater number of embryos for early transfers. Transferring a higher number of embryos results in a higher order of multiple pregnancies and twins with increased pregnancy complications i.e. abortion and premature deliveries
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