We know that the primary reasons embryos don't make it to blast is they don't have the genetic instructions to continue -- and we can't change or improve the genetics or egg quality of an embryo. If you have faith in your lab there really is no risk with continuing to culture those embryos to blast stage.
Some say that embryos are better in the uterus at day 3 while others say they are not because if you were having a baby the old fashioned way the embryo remains in the fallopian tube until day five or blast stage. And really if we think about it (Ask your RE I bet he agrees with me) the uterine environment on day 3 isn't the same as your fallopian tubes. And on top of that with what labs do regarding sequential culture systems that help grow embryos to blast -- those kinds of conditions in the lab are as close to the fallopian tubes as you are going to get. So with all that being said maybe embryos are better off in the lab until blast.
Now -- the reason we grow embryos to the blast stage in the lab is to weed out those embryos on purpose that don't have the genetic potential to grow into babies. So the risk folks talk about I think doesn't really exist. I mean think about it -- if they make it they make it -- if they don't they don't. Now I know there is always the risk that zero embryos make it to blast in the lab but I bet my bottom dollar that the issue is genetically related and not something to do with the lab. ie egg quality or sperm quality issues.
There's also lots of reasons that labs do three day transfers - first of all it's less expensive and cheaper. It's less work for the lab and I think there less liability (i.e. the lab has the embryos for a shorter period of time, the patient always makes it to transfer, and last but not least if the cycle doesn't result in a positive pregnancy test the program still looks fine) So with all that being said I am thinking that day three embryo transfers are done for a myriad of reasons.
So I have to say this about blast transfers -- just because you have a blast transfer doesn't mean you are automatically going to have a baby. Now when your embryo reaches blast stage it's okay to think that these embryos are capable if implanting right where they are supposed to and go on to develop into a baby. There are still a few potential obstacles to overcome -- the embryos are transferred into the uterus and they have to attach to the wall of the uterus and then go on for the next ten days completing the implantation process. That process is out of the control or the hands of the clinic, and like I have always said it's really a crap shoot at that point.
What we do know is at the blast stage embryos are capable of implanting, whereas day three embryos we don't know for sure if they even going to make it to blast -- what we do know however is lots of 3 day transfer result in babies.