What is IVF Single Embryo Transfer?
In recent years, we have made an effort to minimize the risk of multiple pregnancy rates and select a single embryo for transfer. The process of selecting one embryo for transfer involves several factors, since even an embryo that appears microscopically to be of high quality can still have chromosomal errors that can prevent pregnancy from taking place or even lead to pregnancy loss.
In many cases (but not all), testing for chromosomal errors with PGS (CCS) helps us determine the highest-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 blastocyst for transfer on day 5, or have a uterine or cervical factor which makes multiple gestation particularly risky. Studies have found that the pregnancy rate with single embryo transfer are still about 10-15% lower than transferring two embryos.
How Does IVF Single Embryo Transfer Work?
At AFCT, we practice elective single embryo transfer (e-SET). This means that we don’t have predefined criteria, but rather carefully review the patient’s individual circumstances in their entirety before recommending e-SET.
The possibility of e-SET will be discussed with the patient when:
- A woman only has a single embryo available.
- There are several excellent blastocysts available on day 5.
- The woman is under 35 years of age.
- At least one excellent blastocyst is available on day 5 after testing via PGS, and it survived freezing/thawing without any identifiable damage.
- A woman has a history of cervical or uterine abnormalities, which may make a multiple gestation pregnancy particularly risky.
Why does a single embryo transfer result in about 10-15% lower pregnancy rates compared to a double embryo transfer?
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 50%. But that average has very little practical meaning because there are women with nearly 100% good eggs, and there are those with nearly none.
The only proven way to determine egg quality is to fertilize the eggs and to culture them in vitro for 5-6 days. During such extended in vitro culture, the 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.
We have to accept, for now, that for most patients we cannot reduce a chance of a multiple pregnancy without reducing the chance of pregnancy altogether.
Interestingly, the chance of a multiple pregnancy with a single blastocyst transfer is still not zero; about 5% of the blastocysts can split into monozygotic twins. This is a high percentage compared to that of natural conception.
Culturing the embryos to the blastocyst stage, or day 5 or 6 after egg retrieval, may be recommended for patients that have at least 8 fertilized oocytes on the first day after retrieval. The decision to culture embryos to day 5 is based on the development of the embryos and the patient’s individual history. Blastocysts have higher implantation and pregnancy rates, and fewer embryos are usually transferred back to the patient than would be transferred on day 3. Culturing embryos to day 5 may not always be beneficial, as some patients may have embryos that do not tolerate in vitro culture well.
Why Choose IVF Single Embryo Transfer?
Embryo selection using extended in vitro culture to the blastocyst stage is probably one of the most important factors perfected over the last 15 years. Blastocysts can be graded based on their appearance to further improve the selection process.
The chance of a single excellent quality blastocyst to produce a baby may be 50% or higher.
There are also factors other than embryo appearance that determine 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 pregnancy loss.
In many cases (but not all), testing for chromosomal errors with PGS, complete chromosomal testing, (CCS) helps determine the most viable embryo.
Still, even after PGS, the chance of a single embryo becoming a baby is probably not higher than 78%, even in patients with the best prognosis.
Therefore, no matter how much testing is done on embryos, our current ways to select them remain imperfect, and the chances of pregnancy are higher when two or three embryos, rather than just one, are transferred.