The primary reason embryos do not make it to the blastocyst stage is because they do not possess the necessary genetic instructions for continued growth. No one can change or improve the genetics of an embryo. Assuming the lab is competent (and most are), there is no "risk" associated with continuing to culture the embryos to the blastocyst stage. I think you're making the false assumption that the embryos are better off in your uterus on day 3. They're not. Under natural conditions, the embryo remains in the Fallopian tube until the fifth day of development (blastocyst stage). The uterine environment on day 3 is not the same as the Fallopian tubes. When sequential culture systems are employed to grow the embryos to the blastocyst stage, the conditions in the laboratory more closely resemble the Fallopian tubes. Therefore, the embryos are better off in the lab for day 4-5 of development.
The point of growing embryos to the blastocyst stage in the laboratory is to deliberately weed out the embryos that do not have the genetic potential for continued growth. The "risk" you speak of doesn't really exist. If they're gonna make it, they do. If they don't, they don't. Of course, there's always the "risk" that no embryos make it to the blastocyst stage in the laboratory, but (because the problem is related to the genetics of the embryo, not culture conditions in the laboratory) they wouldn't have made in the uterus either.
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You are correct that only the embryos that make it to the blastocyst stage (and beyond) can generate a succesful IVF pregnancy. In my lab, and many others, all embryos are grown to the blastocyst stage and only well developed blastocyst stage embryos are transferred to the uterus on day 5 or 6. Extra embryos are cryopreserved at the blastocyst stage.
Why aren't all programs doing this? There are numerous reasons for continuing to perform day three transfers:
its cheaper, its less work for the lab, lower liability because the lab has the embryos for a shorter period of time, everybody makes it to transfer, if the cycle doesn't result in a pregnancy, the program can still look good, etc. You'll notice I didn't say anything about a day 3 transfer improving your chances of getting pregnant - it doesn't. The reason programs continue to transfer day 3 embryos is because its more convenient for the lab and the docs.
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You're right that many programs will transfer the embryos on day 3 when they are concerned that the embryos may not develop to the blastocyst stage. HOWEVER, its not because they feel the embryos will do better in the uterus. They know there's no scientific evidence to back up this assumption.
It is because they do not want to face the patient and infom them their embryos failed to reach the blastocyst stage. They are afraid that you will think the embryos failed to grow because of suboptimal lab conditions. As I explained, the embryos fail to reach the blastocyst stage because they are genetically incapable of doing so, not because anything anybody did or didn't do. In our program, we attempt to grow ALL embryos to the blastocyst stage. We have at least 1 blastocyst stage embryo for transfer 96% of the time.
You'll notice in your research into the practices of other programs that no programs claim that their pregnancy rates improve when failing embryos are transferred to the uterus on day 3. The rationale for a day 3 transfer is to get out from under the "blame" for the failing embryos. By transferring failing embryos on day 3, the program also transfers the responsibility for the subsequent failed cycle to the patient. It is a subtle manipulation of the patient's emotions. Here's the scenario: "We're so sorry the cycle didn't work, but you know the embryos were still growing when we transferred them. We don't know what you did to them afterwords. Wanna try again?" Using this pyschological manipulation, it becomes the patient's fault the cycle didn't work, not the programs's. See how it works?