Dr Smith replies:
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.
However, growing the embryos to the blastocyst stage prior to transfer does not automatically result in a pregnancy. Because the embryos have reached the blastocyst stage prior to transfer, its reasonable to assume the embryos a capable of implantation. BUT the embryos are transferred to the uterine cavity and they still must attached to the uterine wall (endometrium) and continue the implantation process for 10 ten days before a "pregnancy" is established. The attachement and implantation processes are currently beyond our control and these represent the crap-shoot in all of this.
At least with blastocyst stage embryo transfer you know the embryos are capable of implantation. When day 3 transfers are performed, its completely uncertain as to whether or not the embryos made it to the blastocyst stage (unless there's a pregnancy). If you don't become pregnant, you're left hanging. What went wrong? Were my embryos O.K.? Should I do this again??? Of course, your doc's answer will likely be "Yes" since that keeps him in business. As you can see, there's a potential sinister side to day 3 transfers...