We know that pregnancy rates are better with day 5 embryos rather than day 3 embryos because the chances of a day 5 embryo implanting are better than those of a day 3 embryo implanting. This is because if we grow the embryos to day 5, we allow the embryos to compete amongst themselves so we can pick and choose the best embryos and then select and transfer this. So, if day 5 is good, isn't it logical that day 7 will be even better ? Perhaps we should be growing embryos to a later stage in the lab before transferring them? Intuitively, yes, but there is a problem with this approach.
The truth is there are limitations for how long we can grow an embryo in a lab. Now, this doesn't mean that our in vitro conditions are any inferior as compared to in vivo conditions, especially at least until we reach day 6, because in vitro embryos on day 6 look exactly as good , and are as functionally competent , as in vivo embryos up to day 6. The problem is that by the time the embryo reaches day 7 or day 8, it needs to start acquiring nourishment from the maternal blood supply by burrowing into the trophoblast and establishing a blood supply . It's not possible for us to do this in vitro, that's why our ability to be able to grow embryos after day 7 becomes fairly limited, and this is the reason why we use a day 5 cut-off. The good news is that by day 5 we are fairly good at picking those embryos which have a good chance of implanting and becoming a baby.
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