One of the biggest concerns with in vitro fertilization (IVF) is the risk for multiple pregnancies (twins, triplets, and more). One way to lower the possibility of multiple pregnancy is to transfer only a few embryos, or transfer only one embryo, which is called selective embryo transfer (SET). Blastocyst transfer is a technique used to achieve greater potential for pregnancy.
What is a Blastocyst?
A blastocyst is a developed embryo that has divided several times. When the embryo reaches the blastocyst stage, it is ready to implant into the uterus lining. The blastocyst has a greater potential of successfully implanting and resulting in an ongoing pregnancy because it passes an important test: surviving the first 3-4 days of development.
Survival of the embryo past day 3-4 is significant because during this time, the embryo relies on the egg for all its nutrients. After 3-4 days, the embryo must activate its own genes, which not all embryos are able to do. According to studies, only one-third of all fertilized eggs will progress to blastocysts. Compared to day-3 embryos, day-5 embryos have a higher probability of survival, are stronger and healthier, and are more likely to transfer into the uterine wall.
Significance of the Blastocyst Transfer
It is quite difficult for the embryologist to predict which embryos are likely to produce a pregnancy. In women age 35 years and older, up to three embryos are usually transferred, with the hopes that at least one will survive and produce a live birth. Using this approach, pregnancy rates are higher, with the chance of twins at 35% and the risk for triplets around 8%. Multiple pregnancies present many medical risks to both infants and mothers, as well as serious emotional and financial concerns. Blastocyst transfer only involves 1-2 embryos, which eliminates many of these risks and concerns.
Getting to Blastocyst
In a usual day-3 embryo IVF cycle, the woman’s eggs are retrieved and fertilized, and then the resulting embryos are transferred three days later. Traditional IVF has usually involved transferring day-3 embryos because previous laboratory culture media could only sustain the embryo for up to 3 days. With new technology, embryologists now can develop the embryo to the blastocyst stage, which is a day-5 or day-6 embryo.
Candidates for Blastocyst Transfer
It is not possible for embryologists to predict which embryos will progress into blastocysts. However, they do know that embryos that develop for 5-6 days have a higher chance for pregnancy. In general, clients who develop a large number of eggs benefit most from the blastocyst transfer. Researchers have noted a significant relationship (correlation) between the number of eggs developed and retrieved and the number of blastocysts developed. Candidates for blastocyst transfer include:
Couples who do not wish to have multiple pregnancies.
Women with certain medical conditions where multiple pregnancies increase risks.
Women who develop multiple eggs during ovarian stimulation.
Couples who produce several embryos after IVF fertilization.
What happens when Embryos do not Progress to the Blastocyst Stage?
Around 30% of fertilized eggs go on to develop into blastocysts. Because of this low percentage, it is not always possible to perform a blastocyst transfer. The couple could have no embryos survive to day 5 or 6. For this reason, fertility specialists reserve blastocyst transfer for clients with many eggs, and those who produce good quality embryos. Pregnancy can be achieved with day-5 transfer for select couples, but not all couples qualify for this procedure.