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.
Why a single embryo transfer results 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 30%. But that average has very little practical meaning because there are women with nearly 100% good eggs, and there 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 to further narrow down the most viable embryo.
Still, even after PGS, the chance of a single embryo becoming a baby is probably not higher than 70% even in patients with the best prognosis.
Therefore, no matter how much testing is done on embryos, our current ways to select them remains imperfect, and the chances of pregnancy are higher when two or three embryos are transferred rather than a single one.
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.
Interestingly, the chance of a multiple pregnancy with a single blastocyst transfer is still not zero, because some 5% of the blastocysts split into monozygotic twins. This is a much higher percentage compared to a natural conception.
At AFCT we practice elective single embryo transfer (e-SET). This means that we don't have predefine criteria, but rather carefully review the patient's circumstances in its entirety before recommending e-SET.
A 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, and the woman is under 35 years of age.
- At least one excellent blastocyst is available on day 5 after testing by PGS, and it survived freezing/thawing without any identifiable damage.
- A woman with a history of cervical or uterine abnormalities, which may make a multiple gestation pregnancy particularly risky.
We take the time to get to know each patient individually and will address all your concerns on the road to parenthood in the most compassionate manner.
To learn more about single embryo transfer or to schedule an appointment, please call (713)467-4488.