What is a Blastocyst?
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.
Getting to Blastocyst
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.
Significance of Blastocyst Transfer
One of our greatest concern in In Vitro Fertilization (IVF) treatment is safety; we focus on healthy mother and baby. Since it is relatively difficult to predict on day 3 which embryos are more likely to develop into pregnancy in women, up to 3 embryos are frequently transferred with the hope that at least one will result in a live birth. With this method, good pregnancy rates have been achieved although approximately 35% of these are multiple pregnancies, with up to 8%possibility of triplets. Multiple pregnancies are associated with considerable medical risks to the mother and infants, as well as financial and emotional considerations. Due to the higher probability of survival and implantation, we transfer usually a maximum of two embryos (blastocyst) to prevent higher order of multiple pregnancies and its morbidity and mortality.
Whom does Blastocyst Culture help?
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.
In general, patients who develop a larger number of eggs and embryos benefit most from blastocyst transfer. A significant correlation has been reported between the number of eggs and the number of blastocysts developed, as well as the number of day 3 embryos and the number of blastocysts developed.
In cases of elective single embryo transfer.
In cases where there have been many unsuccessful treatments and in women who are older than 40 years. Culturing to blastocyst stage allows better observation and selection of embryos for transfer.
When Blastocyst Culture is not considered beneficial?
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:
- Who have many eggs
- Who have more than 5 embryos on day 3
- Who have elected to have blastocyst transfer regardless of the number of eggs
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.