It has long been recognized that as embryos progress developmentally during the first 5-6 days following fertilization, many of those of “poorer quality” succumb along the way. It follows that a much higher percentage of embryos that survive to the fifth or sixth day post-fertilization is likely to be of “good quality” than would be the case by day two or three. IVF researchers applied this principle in an attempt to achieve high pregnancy rates without increasing the risk of high order multiples. They did so by challenging embryos to progress to their most advanced pre-implantation stage of development (i.e. the blastocyst stage) by day five or six post-fertilization and then selecting only one or two such “better quality” embryos for transfer.
The concept of blastocyst transfer (BT) is not new to the field of Assisted Reproduction. In fact, reports of human pregnancies following BT date back to the early 90’s. However the ability to consistently produce a high percentage of blastocysts from cultured embryos is a relatively recent development.
The freshly fertilized egg before it starts to cleave (divide) is called a zygote. After the first 24 hours, (Day 1) the embryo has divided into 2 cells. By Day 2 the embryo has 4 cells (blastomeres) and by the third day, there should be between 6 and 9 cells. Up to that point, embryonic development is under the control of maternal genes in the egg. Around the 8 cell stage the embryo’s own genes (embryonic genome) begin to take over control of development. By the fourth day the embryo has between 16 and 32 cells. At this point it looks like a mulberry and is called a morula. Until the morula stage all the embryo’s cells are the same and are totipotential (i.e. they have the ability to ultimately develop into any tissue or organ type). By day 5 post-fertilization, differentiation of the embryo begins. A fluid-filled cavity (blastocele) forms in the center of the conglomerate of blastomeres. This blastocele will eventually become the amniotic sac and the fluid surrounding the conceptus in the uterus. The cells around the outside of the morula develop into the trophectoderm, which will eventually form the placenta and fetal membranes, while the cells on the inside of the morula aggregate and group together to form the inner cell mass, which ultimately develops into the fetus. This complex creation is now called a blastocyst. As the blastocele fills with fluid, the blastocyst expands, its walls thin out and it eventually breaks through (hatches) its envelopment (zona pellucida). The trophectoderm then begins to invade the uterine lining (implantation) by the 6th to 8th after ovulation or egg retrieval (when IVF is performed).
Human embryos are very fastidious. They have specific metabolic requirements in order to survive. The earliest type of artificial embryo culture media developed for IVF purposes was relatively simple in composition and could only support limited embryonic development in the Petri dish and incubator. Thus, the majority of embryos cultured in such media could only survive to the third day whereupon their development would arrest. Subsequent improvements in media composition allowed for more reliable embryo development to the third day… which became and remained the standard time at which embryos were transferred for more than two decades.
Starting in the mid-1990s, researchers in Australia, Scandinavia and in the USA simultaneously developed a new generation of culture media that can support the growth of embryos to the fifth or sixth day. This development was based on the recognition that the metabolic needs of the early embryo changes as it progresses in development, much in the same way as happens in natural conception as the embryo journeys through the Fallopian tube to the uterus. These so-called “sequential culture systems” are improved embryo culturing methods designed to simulate what happens in the reproductive tract during natural conception. They involve sequentially changing the media environment as embryo development progresses. Approximately 35-40% of “good quality” day-3 embryos (comprising 6-9 cells with minimal/no fragmentation) can be grown to the blastocyst stage using such advanced culturing methods.
It is important to note that in spite of the introduction of specialized sequential culture systems and other new techniques, at best 35-40% of “good quality” embryos develop into blastocysts (even in younger women who produce the best quality eggs). However, since blastocysts are more likely to implant than are day-2 or day-3 “good quality embryos”, the transfer of but a few good quality blastocysts yields better IVF success rates than would be the case following the transfer of a higher quantity of early embryos. Moreover, by transferring fewer blastocysts, the fertility clinic can substantially reduce the risk of high order multiple pregnancies.
The presumption that early embryos would be better off in the uterus than would be the case in an incubator in an IVF laboratory is erroneous. This fact was established conclusively through a study we recently performed and published on in the journal Fertility and Sterility in 2007. This research involved genetically testing each of a large number of eggs sequentially, using comparative genomic hybridization (CGH); first, prior to fertilization, and then twice more in turn (on day 2 and day 3). We then followed each of the resulting embryos in culture to day 5-6 to see if they would develop into blastocysts. The study revealed that a high percentage of the embryos that developed to blastocysts had originated from chromosomally normal (euploid) embryos and were thus “competent” (highly likely to develop into normal babies) while with few exceptions, those that did not develop into blastocysts were almost invariably chromosomally abnormal (aneuploid) and were thus “incompetent”. We concluded that since the uterine environment is no more favorable than the Petri dish in terms of embryo development, there is no advantage in transferring embryos to the uterus earlier than the blastocyst stage.
The use of CGH to select the most competent embryos for transfer, requires that they first be grown to blastocysts whereupon they be cryopreserved (frozen) for days or weeks until the results of CGH testing are available. Thereupon, one or two euploid (“competent”) embryos are transferred to the uterus. This modified application involving CGH selection of the “best” blastocysts to transfer has further enhanced the efficiency of IVF, markedly increased the baby rate per embryo transferred, reduced the miscarriage rate, and minimized the occurrence of chromosomal birth defects.
It is my personal opinion that with few exceptions, blastocyst transfer should be performed preferentially in IVF. Other than convenience and easing pressure on doctor and/or patient, there is in my view seldom any advantage in transferring embryos on day 2 or day 3. After all, we have demonstrated conclusively that embryos failing to survive to the blastocyst stage are almost certainly (aneuploid) “incompetent,” and are thus unable to propagate normal pregnancies anyway.
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