Single Embryo Transfer: One Embryo, One Baby

With the evolution of new and advanced techniques of in vitro fertilization, resulting in improved embryo implantation and birth rates, there has come about a substantial increase in the rate of multiple births. Largely as a result of this, obstetrical outcomes are poorer following IVF than following spontaneous pregnancy. In fact, multiple births are responsible for a markedly higher risk of prematurity with low or very low birth weight, as well as perinatal death and more frequent lingering neurologic complications as well as an increased risk of malformations.

Consider the fact that when comparing singleton with triplet pregnancies:
• Twins have 3-times, and triplets, a 6-times greater perinatal mortality rate
• Twins have 6-times, and triplets, an 11-times greater likelihood of developing cerebral palsy
• Twins are 50%, and triplets 80% more likely to be born prematurely
• Mothers of twins are 3-times, and mothers of triplets, 7-times more likely to experience serious pregnancy-induced complications.

The anguish of losing one or more of your children at birth or watching them endure a life-long disability is a situation no parent would wish to face, yet it is a frequent consequence of multiple births. So why then do so many IVF practitioners still insist on transferring multiple embryos at a time? The following are the main reasons for this:

• Most infertile patients simply do not perceive any great risk associated with multiple gestations, especially when it comes to twins. In fact most, consider multiple pregnancy to be a “bonus”…a favorable outcome. Faced with the high emotional and financial cost associated with IVF treatment, most couples prefer to complete their families in one attempt so as to “maximize the use of their resources.” In fact, when asked, almost 90% of couples undergoing IVF in the United States are desirous of having twins. Some are even interested or covet having high order multiples (triplets or beyond). Education is urgently needed to make IVF candidates fully aware of the risks associated with multiple gestations.
• The relative inability hitherto, to reliably differentiate between embryos that will propagate a healthy pregnancy (i.e. “competent” embryos) and those that will not (“incompetent” embryos): Most IVF patients erroneously believe that a “pretty”, embryo (one given a high grade because it fulfils the microscopic criteria of “good quality”) should invariably propagate a baby. This is simply not the case. Consider the fact that such a microscopically “good quality” embryo from a 30-year-old has about an 8-times greater chance of resulting in a normal birth than would an identical looking embryo of a 45 year old! This confronts IVF practitioners with a “damned if you do, damned if you don’t” situation; driven by patient pressure to achieve a pregnancy and by competing market forces, they still too often choose to transfer multiple embryos, often with disastrous results. It is generally true that declining egg / embryo “competency” with advancing age justifies transferring more embryos in older women - especially in those over 40 years of age - but this still needs to be carefully measured against the risk of multiple gestations.

Not only is multiple gestation the most common complication of infertility treatment, it has also become the most costly in terms of its social impact. If all of the factors associated with multiple gestations are considered, including the costs of antenatal maternal hospitalization, neonatal intensive care for premature infants, as well as the costs of chronic medical care, rehabilitation and special education, the projected annual cost of IVF-associated multiple gestations in the United States is approximately $1.5 billion as compared to about $550 million for all the other IVF cycles performed.

On the positive side is the fact that the last decade has seen a slight but significant decline in the IVF twin pregnancy rate from about 25% to about 22%, as well as a decline in the incidence of triplets from 5% to about 3%. Still, IVF multiple birth rates are about ten times higher than those associated with natural conception. Clearly, multiple pregnancy (especially high-order multiples) represents a complex problem that can no longer be justified as an acceptable outcome following IVF treatment.

Numerous studies have demonstrated that the cumulative birth rate after single embryo transfer (SET), followed by subsequent transfers of individually thawed left-over embryos, is as effective in achieving pregnancy as implanting multiple embryos at one time. And by this approach the risk of multiple births can be virtually eliminated. Moreover, using the SET approach, more than 80% of women <40>In the past, the ability to select competent embryos for transfer has been thwarted by:

a. Lack of reliability of microscopic morphologic (appearance) embryo grading.

b. The inability of traditional pre-implantation genetic diagnosis/sampling (PGD/s) and chromosomal evaluation (karyotyping) by conventional Fluorescence In-Situ Hybridization (FISH) to be able to access all of the embryo’s chromosomes

Let’s take a look at the merit and reliability of several current methods used to select the best embryo(s) for transfer:

• Microscopic Embryo Grading: Currently, most IVF centers culture embryos in groups and then perform a single microscopic evaluation (at 2, 3 or 5-6 days) prior to transferring one or more to the uterus. This approach is limited in scope and in its ability to reliably discriminate between "competent" and "incompetent" embryos, since chromosomally abnormal embryos are often identical in appearance to those that are normal. Embryos should be at 2 to 4 cells at 48 hours after egg retrieval and about 6-9 cells by 72 hours. The cells in an embryo are also referred to as “blastomeres". Ideally the blastomeres should be of even size, and there should be less than 20% fragmentation, or blebbing. This is where portions of the embryo's cells have broken off and are found lying free as debris inside its substance. Most IVF clinics "grade" each embryo using one of many scoring systems. Unfortunately, there is no agreement at all as to which system to use. But regardless of the microscopic grading system used, one thing is certain…they all lack reliability because they cannot evaluate the chromosomal integrity of the embryo.

• Blastocyst Embryo Transfer: A blastocyst is an embryo which has developed to the point of having 2 different cell components and a fluid cavity. Human embryos, in culture in an IVF lab, or developing naturally in the female body, usually reach the blastocyst stage by day 5 or 6 after fertilization. Many “incompetent” embryos are culled out as the embryo progresses to the blastocyst stage. Thus, those embryos that make it to blastocyst are much more likely than their day-3 counterparts to be competent. Embryos that do not reach blastocyst are in >95% chromosomally abnormal (aneuploid) and would not have been worthy of transfer earlier on anyway. Routinely taking embryos to blastocyst is thus a good idea since if they do not make it they are incompetent anyway.
  Following fertilization, the cells of the embryo divide progressively over several days until after 4 days the embryo reaches an advanced (100 cell or more) stage known as a morula. One or two days later the morula will have differentiated further, developing a defined fluid filled cavity within its structure. Only about 40% of a younger woman’s embryos make it to this highly advanced or blastocyst stage of development. With few exceptions, embryos that fail to progress to the blastocyst stage are in fact aneuploid and therefore incompetent. By waiting five or six days post fertilization to select and transfer only blastocysts to the uterus, we can improve the likelihood that those being transferred are more likely to be the 'competent" ones.

• Conventional Fluorescence In-Situ Hybridization (FISH): FISH is a method used to identify up to 12 of the 23 chromosome pairs in the embryo for abnormalities. The process requires removal of one of the 3-day old embryo’s cells (blastomeres) by a process referred to as Pre-Implantation Genetic Diagnosis/Sampling (PGD/s). Unfortunately, the remaining chromosome pairs cannot be accessed by conventional FISH and attempts to perform 23 chromosome FISH still lack sensitivity and specificity. In fact there remains about a 45% likelihood of an aneuploidy involving one or more of the remaining chromosomes - even when conventional FISH results are reported as “normal”.

• Comparative Genomic Hybridization (CGH): This very promising method for egg/ embryo selection was introduced into the clinical arena by SIRM in 2005. It represents a real break through in the IVF arena. Unlike conventional FISH testing which can only reliably recognize 12 of the embryo’s 23 chromosome pairs, CGH allows for identification of ALL the chromosomes and in the process, overcomes the inadequacies associated with most other methods of embryo selection. A study we published in Fertility & Sterility in May, 2005 demonstrated a birth rate of more than 70% in women who received just one CGH-selected embryo. Our follow-up study reported in Fertility and Sterility (December 2009) confirms that through CGH embryo selection we finally have a highly reliable method for differentiating between “competent” and “incompetent” embryos. Even without CGH embryo selection, “one embryo/one healthy baby” is now even more attainable. However, the introduction of CGH has made embryo selection much more scientific, virtually removing the incentive to transfer multiple embryos at a time.
  One of the perceived disadvantages of CGH embryo selection is the associated increase in cost of such testing. However, while the performance of egg/embryo CGH does increase the cost per cycle of IVF, it actually lowers the “cost per IVF baby”.
  Since CGH testing requires several days or weeks to complete, the use of this technology usually requires that advanced embryos (blastocysts) be frozen and stored (cryostored) in a subsequent cycle . The separation of an IVF cycle is separated into two separate phases to achieve this objective is referred to as Staggered-IVF (St-IVF). Cryostoring blastocysts allow sufficient time for the CGH testing to be completed.
  
• Vitrification (Ultrarapid Freezing): Until recently cryopreservation of human embryos has been problematic because it often caused ice crystals to form inside the embryo, damaging or destroying it. The recent introduction of ultra-rapid freezing or vitrification (see the article on vitrification elsewhere on this blog) has changed all that. With vitrification, embryos are so rapidly frozen that no ice forms, yielding a post-thaw embryo survival rate of more than 95%. Impressively, birth rates following the transfer of thawed, previtrified embryos hardly differ from those using fresh embryos.
  

The Hippocratic Oath, decrees that the cardinal rule of medicine is “primum non nocera (”foremost do no harm”). Since multiple pregnancy is the most serious complication of Assisted Reproductive (AR) Medicine, and IVF has been responsible for a virtual explosion in the incidence of twins and higher order multiples, those of us that practice medicine in this arena have a solemn responsibility to educate our patients and then to restrict the number of embryos we transfer at one time. Central to achieving this goal is to optimize the ability to select the most “competent” embryos for transfer.

The transfer of a single, good quality embryo selected using objective methods will be a central part of IVF treatment in the years to come. In contrast to many areas of AR Medicine and infertility where additional evidence is often sought and further trials are warranted, we believe there now is sufficient information to start moving to SET, especially good-prognosis women who are undergoing IVF treatment.