Reprinted with permission from Atlanta Reproductive Health Centre, Atlanta, Georgia
In this article, Mark Perloe, M.D., E. Scott Sills, M.D., and Carolyn Kaplan, M.D. of the Georgia Reproductive Specialists, and Michael Tucker, Ph.D. offer a comprehensive look at blastocyst transfers. Dr. Tucker and his team of embryologists are at the forefront of blastocyst research, working diligently to maximize pregnancy rates while minimizing risks.
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The first thing that usually comes to mind when people hear the term, "infertility treatment," is the risk of multiple births. This worry has been fueled by the recent highly publicized multiple births in Iowa and Texas. While such cases are rare, the incidence of triplets or greater as a result of assisted reproductive technology is of great concern to all infertility practitioners and patients. For countless couples, deciding against treatment for this reason may mean abandoning their dream of having a child.
But what if there were a way to reduce or even eliminate the risk of multiples? Not only would that help more couples become parents, it would also substantially decrease maternal and neonatal risks. Such a possibility is becoming a reality, thanks to a new technique known as blastocyst transfer. With blastocyst transfer, fewer embryos are transferred while maintaining or even increasing pregnancy rates. This technique virtually eliminates the risk of triplets or greater.
The Significance of Blastocyst Transfer
In a traditional non-blastocyst in vitro fertilization (IVF) cycle, a woman's eggs are retrieved and fertilized. If all goes well, the resulting embryos are transferred into the uterus three days later. But because it is difficult to predict which day three embryos will become an ongoing pregnancy, three or more embryos are usually transferred in hopes that at least one will implant and result in a live birth. Until now, this seemed a reasonable approach in order to achieve acceptable pregnancy rates.
The downside with typical day three transfer is that sometimes all the embryos become ongoing pregnancies and the result is high-order multiple gestations (triplets or greater). In such pregnancies, there are considerable medical risks as well as financial, emotional, and social considerations. So the couple is faced with the agonizing decision of whether to opt for selective reduction (the removal or destruction of one or more fetuses) or to continue with a risky pregnancy. Although everyone agrees that every possible safeguard should be in place to avoid such unfortunate situations, the distressing reality is that multiple gestations will occur.
However, with blastocyst transfer, fewer embryos are transferred, practically eliminating the possibility of triplets or greater. And the same pregnancy rates are achieved as would be expected when four or more embryos are transferred on day three. Indeed, some centers report achieving even better pregnancy rates with blastocysts. Implantation rates of 48-50% and pregnancy rates of up to 66.3% have been reported in patients who responded well to gonadotropins and received transferred blastocysts .
What is a Blastocyst?
A blastocyst is a highly developed embryo (usually developed by day five or six after fertilization) that has divided many times over to a point where is composed of 100 cells or more. At this stage, it is nearly ready to implant on the walls of the uterus. A blastocyst has come a long way from its beginning as a single cell, and is much more complex that it was even at day three (typically at the 8-cell stage).
During maturation, an embryo rests inside a protective shell called a zona pellucida. You can think of this protective shell as being much like a chicken egg. But, unlike chicken eggs, human embryos do not remain inside their shells. Instead, the embryo hatches (breaks out of the shell) on the fifth or sixth day so it can attach to the uterine wall (implantation). Just prior to hatching, an embryo becomes a blastocyst.
Interestingly, we have noted that the "slower" embryos (those requiring six days to form a blastocyst) seem to possess a lower potential to hatch naturally from their own zona pellucida. Recently, the hatching potential and implantation rates for these day six blastocysts have been improved through the use of assisted hatching performed in the IVF lab .
Embryos developing to the critical blastocyst stage have a much greater chance of implanting successfully and resulting in an ongoing pregnancy. That is because these embryos have passed an important test. During the first few days, the embryo relies on the mother's egg for much of its nutrients and developmental instructions (genetic coding). However, in order to survive past day three, the embryo must activate its own genes. Not all embryos are successful in making this critical shift. In fact, only about one-third to one-half of the embryos become blastocysts. Yet these embryos are more developmentally advanced, healthier, and stronger - and have a significantly higher rate of implantation - when compared to day three embryos. And because they have a higher probability of surviving, we can transfer fewer without reducing the chance of pregnancy.
Getting to Day Five
Infertility practitioners have known for many years that day three transfers were too early when compared to what is physiologically normal. In naturally conceived pregnancies, a day three embryo resides in the fallopian tube, not in the uterus. The embryo does not even reach the uterus until the fifth or sixth day. Yet traditional IVF has always transferred embryos to the uterus on day two or day three because, up until now, we have not been able to consistently delay the transfer to day five or six. Even the world's first test-tube baby in 1978 resulted from a "day 2.5" transfer. Previous laboratory culture media could only sustain an embryo's growth for three days. But now, we have the ability to develop an embryo to the blastocyst (day five/six) stage on a routine basis.
What has made the difference is the recognition that the nutritional requirements of the embryo changes as it develops. That knowledge led to the development of different laboratory culture media for the embryo's specific developmental stages. These so-called "sequential media" attempt to reproduce the natural environment of the maternal reproductive tract. The nutrients are designed to meet the changing needs of the rapidly developing embryo and have led to the growth of blastocysts with better viability and improved implantation potential.
Redefining Developmental Potential
The ability to develop embryos to the blastocyst stage allows clinicians to have greater certainty about which embryos are more likely to implant. Interestingly, only a very weak correlation has been found between what is traditionally considered a "good embryo" on day three and a "good blastocyst" on day five/six. Dr. Tucker reported a "significant disparity (up to 48% errors) between the two stages in embryo viability estimates ," meaning that even the best embryologists cannot always tell which day three embryos have the potential to develop into blastocysts.
While blastocyst quality is determined by examining morphology and development, it is important to point out that blastocyst grading standards are currently still being developed. Although the ability to grow into a blastocyst is a milestone for an embryo, other factors also play a role in its further development. In the near future, we believe we will be able to predict more accurately which blastocysts are destined for success. Eventually, single blastocyst transfers will be considered the norm, and IVF will likely be considered the first-line infertility treatment.
Which Patients Benefit?
Determining who is a good candidate for blastocyst transfer is another rapidly evolving area. As more information becomes available and our knowledge base grows, guidelines based on actual clinical experience will be developed. Until then, we can only offer some preliminary observations.
In general, blastocyst transfer is more advantageous for patients who develop a large number of oocytes and embryos. A significant correlation has been reported between the number of oocytes and the number of blastocysts developed, as well as the number of day three embryos and the number of blastocysts developed . Other candidates for blastocyst transfer include those who would not consider fetal reduction or those in whom delivering multiple pregnancies would be particularly unsafe. Blastocyst transfer is probably not advantageous for patients who develop only a few oocytes or embryos, unless they wish to utilize extended culture to the blastocyst stage as a diagnostic indicator of embryo quality.
A side benefit of a blastocyst transfer is the fact that the ability to generate a blastocyst provides important information about the likelihood of pregnancy. In general, pregnancy rates are higher in those whose embryos grow to the blastocyst stage. Conversely, pregnancy rates are lower in those whose embryos do not develop into blastocysts. At present, clinicians cannot intervene to help a day three embryo become a blastocyst. Extended culture, therefore, does not improve embryo quality. Currently, it merely reveals it. However, the future will see development of even better culture techniques enabling us to improve the chance that day three embryos will progress to blastocysts.
Maternal Age and Blastocyst Development
Does maternal age have any bearing on the production of blastocysts? Although some studies have shown advanced maternal age to be negatively correlated with blastocyst production, Schoolcraft found "no correlation between percentage of blastocyst formation and increasing maternal age" in a population of women who responded well to gonadotropins . Nevertheless, implantation rates and pregnancy rates decreased with increasing maternal age, such that outcomes were worse in women over 40 [1,4].
When Good Embryos Turn Bad
In some instances, only a few embryos develop to the blastocyst stage. In fact, it is possible to have no embryos survive that are suitable for transfer on day five or six. This is especially true if the cycle begins with only a few fertilized eggs. When no embryos survive to become blastocysts, it is a tremendous disappointment. The looming question then arises, "Would the embryos that did not survive to become blastocysts have implanted if transferred at day three?" Unfortunately, we simply do not have enough clinical data at this time to answer that question. In our opinion, pregnancy would have been unlikely in that situation. But since that outcome is not a certainty, day three transfers may still be a reasonable option for some patients.
Ultimately, everything centers around the level of confidence one has in the in vitro laboratory culture conditions. Does this artificial environment provide conditions compatible with those experienced in the uterine cavity? In reality, this comparison may vary from couple to couple, depending on factors beyond the control of the laboratory or the physician.
Genetic Testing and Blastocysts
Another benefit of blastocyst transfer is the ability to perform biopsies on a more highly-developed embryo in order to test for genetic diseases. In the future, micro-testing techniques (including PCR, flourescent PCR, genomic hybridization and flourescent in situ hybridization) will allow practitioners to remove a few cells from an 8-cell embryo, stain them, examine them under the microscope to detect any genetic anomalies, and transfer only apparently normal blastocysts two days later. While that type of testing is not currently available on a routine basis, we believe it will be considered routine within the next two to five years.
Frozen Blastocyst Cycles
Good quality blastocysts tend to have a very good survival rate after cyropreservation* (freezing). Menezo et al. have reported that "the recovery after thawing is equivalent if not superior to that after thawing of earlier embryonic stages .
Since blastocysts are superior to earlier stage embryos in terms of development, they represent a more advanced and potentially viable embryo to freeze, store, and thaw. Additionally, because blastocysts have higher implantation rates, fewer are needed for transfer. This means there may be more left over for freezing, possibly allowing a couple to go through IVF once and have enough blastocysts for the current cycle as well as any future thawed-embryo transfer cycles.
We are just beginning to understand the implications of blastocyst transfer for both practitioners and patients. We believe infertility treatment centers will soon be able to reliably grow blastocysts and accurately assess which embryos are destined to implant and develop into an ongoing pregnancy. With that level of confidence, the transfer of a single blastocyst will become the norm and today's risk of high-order multiples will become a fast-fading memory. The future holds much hope, much promise, and considerably fewer risks. It is a very exciting time to be involved in IVF therapy.
 Schoolcraft WB, Gardner DK, Lane M, et al. Blastocyst culture and transfer: analysis of results and parameters affecting outcome in two in vitro fertilization programs. Fertil Steril 72:604-9,1999.
 Tucker MJ et al. Day three morphology is a poor predictor of blastocyst quality in extended culture. Submitted Fertil Steril Dec 1999.
 Tucker M: Relevance of assisted hatching with blastocyst transfer. Proceedings The First Congress on Controversies in Gynecology and Obstetrics and Infertility, Prague CZ, October 1999.
 Janny L, et al. Maternal age effect on early human embryonic development and blastocyst formation. Mol Reprod Dev 45(1):31-7,1996.
 Menezo YJR, Kauffman R, et al., A mini-atlas of the human blastocyst in vitro, Zygote 7:61-65,1999
Copyright © 1999 by Mark Perloe M.D., P.C. Atlanta, GA, USA.