What is a Blastocyst?
A blastocyst is a 5 – 6 days 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 (the womb). It has a greater chance of implanting successfully and resulting in an ongoing pregnancy because it has passed one very important stage, 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 a traditional In Vitro Fertilization (IVF) cycle, a woman’s eggs were retrieved and fertilized. If all went well, the embryos were 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.
The Significance of a Blastocyst Transfer
One of our greatest concerns in IVF treatment is safety; we focus on a healthy mother and a healthy baby. Since it is relatively difficult to predict on day 3 which embryos are more likely to develop into pregnancy in women, 3 embryos were frequently transferred with the hope that at least one would result in a live birth. With this method, good pregnancy rates were achieved witch approximately 35% being multiple pregnancies, with an up to 8% possibility of triplets. But multiple pregnancies are associated with considerable medical risks to the mother and infants, as well as the financial and emotional considerations. Due to the higher probability of survival and implantation, we now usually transfer only a maximum of two embryos (blastocysts) to prevent a higher order of multiple pregnancies.
Who does Blastocyst Culture help?
It is not possible to predict which day 3 embryos will develop into blastocysts but embryos that develop into blastocysts have a higher chance of leading to pregnancy.
• In general, patients who develop a larger number of eggs and embryos benefit most from a 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 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 offer 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 pertinent question many people have asked is, “Would the embryos that did not survive to become blastocysts have implanted if transferred at day 3?” Unfortunately, we cannot answer this question definitively. However, in our opinion, pregnancy would have been unlikely. But since such an outcome is not a certainty, traditional day 3 transfers are still reasonable for patients with a few good quality embryos on day 3.