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A blastocyst is a 5 – 6 day old, highly developed, embryo that has divided several times to a point where it is nearly ready to implant onto the walls of the uterus (womb). It has a greater chance of implanting successfully and resulting in an ongoing pregnancy because it has passed one very important test, which is survival beyond day 3. Survival after day 3 or 4 is important because embryos rely on their mother’s eggs for all their nutrients during the first 3 days when they are just at 8-10 cell stage.
However, in order to survive past this time, the embryo must activate its own genes; not all embryos will successfully transit to this stage of development. Blastocysts are embryos that have a higher rate of implantation when compared to day 3 embryos. Transferring embryos at blastocyst stage mimics what happens naturally, as embryos arrive in the uterus to implant at this stage of development after fertilization in the fallopian tube.
Until recently, in traditional IVF cycle, a woman’s eggs are retrieved and fertilized. If all goes well, the embryos are transferred into the uterus 3 days later because earlier laboratory culture media (the nutrient solutions in which the embryo survives on outside the uterus) could only sustain an embryo’s growth for 3 days.
We now have the ability to culture an embryo to the blastocyst (day 5 or 6) stage. This has been made possible by advanced culture systems and improved technology to support blastocyst culture.
It is not possible to predict which day 3 embryos will develop into a blastocyst but embryos that develop to blastocysts have a higher chance of leading to pregnancy.
Blastocyst transfer is probably not beneficial for patients who produce few eggs or few embryos. Given that only a fraction of fertilized eggs develop to the blastocyst stage, it is possible to have no embryos survive to day 5 to transfer. This is why we reserve offering blastocyst transfer to patients:
The imminent question many people have asked is that, "Would the embryos that did not survive to become blastocysts have implanted if transferred at day 3?" Unfortunately, we cannot affirmatively answer this question. However, in our opinion, pregnancy would have been unlikely in that situation. But since that outcome is not a certainty, traditional day 3 transfers are still reasonable for patients with few good quality embryos on day 3.