The goal of every IVF doctor – and every IVF patient – is to optimize IVF success rates. Because there are so many variables in the IVF process, we as physicians and scientists systematically approach and address each one of these in an attempt to optimize outcome. One such variable that has been debated for many years is the optimum timing of the embryo transfer. Previously (and to a large extent even today), IVF doctors and embryologists intuitively believed that since the uterus provides the “natural” environment for embryo development while the Petri dish and incubator do not. Thus, the sooner an embryo was transferred to the uterus, the better off it would be. Accordingly, an embryo transferred as on day 2 -3 post-fertilization rather than as a blastocyst (on day 5 or6) would have a better chance of resulting in a live birth. We now know this belief to be erroneous.

Background information: 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 of 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. By this stage the embryo comprises more than 100 cells.

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 day after ovulation (or egg retrieval in the case of IVF treatment).

Human embryos are very fickle. 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 the USA simultaneously developed a new generation of culture media that could support the growth of embryos to the fifth or sixth day. This development was based on a clearer appreciation of the metabolic needs of the early embryo. Now, approximately 35-40% of day-3 “good quality” embryos (comprising 6-9 cells with minimal/no fragmentation) can be grown to the blastocyst stage using such advanced culturing methods

A strong argument in favor of routinely culturing embryos to the blastocyst stage: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. A few years ago we reported on the fact that embryos failing to reach the blastocyst stage are almost always aneuploid (have an irregular chromosome component) and are thus “incompetent” (i.e. unable to propagate a viable pregnancy). However, the converse does NOT apply, that is, not all embryos surviving to the expanded blastocyst stage are “competent” embryos. What it does mean is that since those embryos that arrest in development prior to reaching blastocyst are “incompetent” the further the embryo develops, the more likely it is to be “competent”.

It is important to note that in spite of the introduction of specialized culture systems and other new techniques, at best (even in younger women) 40% of “good quality” day 3 embryos (those that are 6-9 cells and have little fragmentation) will develop into expanded blastocysts by day 5-6 post-fertilization in younger women. This percentage drops dramatically as women age beyond 39 years. Since blastocysts are much more likely to implant following embryo transfer (ET), the transfer of 1-2 good quality blastocysts will yield a far better IVF success rate than would the transfer of a higher number of earlier embryos. In addition, by transferring fewer blastocysts, the risk of multiple pregnancies is significantly reduced.

So, a compelling body of evidence has finally brought us to the point that we now recognize that embryos failing to develop in the laboratory to the expanded blastocyst (advanced preimplantation) stage by day 5-6 post-fertilization, would not have developed into a viable pregnancy had they been transferred to the uterus earlier on in the cleaved stage (day 2-4 post-fertilization). We further appreciate that the rate of development of an embryo in the embryology laboratory provides a significant indication of its likelihood to subsequently develop into a blastocyst by day 5-6 post-fertilization and also of its ability to propagate a viable pregnancy. This prognostic ability has been aided by new embryology laboratory techniques as well as by the development of more sophisticated embryo culture media that more closely mimic the uterine environment.

The new found ability to safely freeze blastocysts using a new ultra-rapid process known as vitrification (which does not compromise their subsequent (post-thaw) ability to propagate a viable pregnancy, has led to many IVF programs to preferentially cryobank vitrified blastocysts for transfer in one or more subsequent frozen embryo transfer (FET) cycles. This has been shown to be especially helpful in older women (and those with diminished ovarian reserve) who in an effort to” make hay while the sun shines” choose embryo banking (stockpiling) in an effort to reduce the ravaging effects of a rapidly advancing “biological clock.” Also, the recent introduction of genetic embryo competency testing (using methods such as CGH) demands that the best embryos (blastocysts) be cryobanked while awaiting the results of the testing so as to select “competent” embryos for future FET. For all these reasons (and there are probably many others) most IVF centers of excellence have graduated from transferring cleaved embryos to blastocyst transfers.

Other than patient preference and easing pressure on doctor and/or patient, there is in my view seldom a justification for transferring embryos on day 2 or day 3. In light of the aforementioned revelations and developments I therefore hold that blastocyst transfer should be the embryo transfer method of choice.