Elective Single Embryo Transfer (e-Set)
During an IVF cycle, the physician and the embryologist have to strike a delicate balance between giving a patient the best chance of pregnancy while limiting the chance of multiple pregnancies. In the ideal world, we would like to fertilize only a single egg, transfer a single embryo and have a single baby. This is because a singleton pregnancy is best for a safe and uneventful pregnancy for the mother and the full-term development for the child.
However, limiting the number of eggs to be fertilized, as, for example, done in Italy, results in a very low pregnancy rate of about 8% per cycle. In other words, only 8 couples out of a 100 starting an IVF cycle will have a baby. This is about five times lower than in the United States. With such low level of effectiveness, IVF treatment would not be affordable for the majority of couples who need it. The answer is as simple as it is vague: not every egg is able to develop to term once fertilized. We call this phenomenon a variation in egg quality. The majority of eggs from an average woman are simply not good enough to produce a viable embryo.
Depending on the woman’s age and other factors, the percentage of “good” eggs is between 5% and 30%. (That average has very little practical meaning because there are women with nearly 100% good eggs, and they’re those with nearly none.) The only proven way to determine egg quality is to fertilize them (all eggs) and to culture them in vitro for 5-6 days. During such extended in vitro culture, those embryos that are not viable will stop developing, while those that are viable will continue and become a so-called blastocyst by day 5 or 6. Embryo selection using extended in vitro culture to the blastocyst stage is probably the most important instrument perfected over the last 15 years in IVF laboratory. Blastocysts can be graded based on their appearance, to further improve the selection process. The chance of a single blastocyst of the excellent quality to become a baby once transferred into the uterus may be as high as 50% or even slightly more.
There are also factors other than merely embryo appearance that determining viability. Even if an embryo looks normal, it may still have chromosomal errors preventing pregnancy from taking place. Such chromosomal errors are frequently responsible for a pregnancy loss.
In many cases (but not all), testing for chromosomal errors with PGS (CCS) helps us determine the best quality embryos which can lead to the highest pregnancy rates. Other factors in favor of single embryo transfer are when a patient is under the age of 35, have several excellent blastocysts for transfer on day 5 or have a uterine or cervical factor, which makes multiple gestations particularly risky. Studies have found that the pregnancy rate with single embryo transfer is almost as good as two embryos.
During an IVF cycle, the physician and the embryologist have to strike a delicate balance between giving a patient the best chance of pregnancy while limiting the chance of multiple pregnancies.
