After fertilization, the embryo goes through several stages of development. After 5 or 6 days, it is called a blastocyst. In the case of natural conception, during this phase the embryo normally leaves the fallopian tubes and enters the uterus, where it is implanted. In the case of IVF, however, many clinics offer the possibility of transferring the embryo when it reaches the blastocyst stage. This means that the embryos develop in the laboratory until day 5 and then, they are transferred to the uterus. Embryo transfer is the final step in the IVF process. Nine days after blastocyst embryo transfer the pregnancy test will be positive in case the fertility treatment is successful.
What is the best day for embryo transfer?
Embryo transfer is possible at any time, from day 1, when it is estimated to be fertilized, to day 6 or 7. However, embryo transfer is usually carried out on day 3 or 5, as these are the times when the quality of the embryos is easy to analyze and assess.
Transfer of embryos at blastocyst stage in IVF
Embryo transfer at day 5 is not something new, either as a concept or as a technique. It has been systematically performed by various groups using co-culture technology since 1992. But what are the advantages of blastocysts?
- Better embryo selection thanks to prolonged cultivation. Development varies from embryo to embryo, and it is known that the implantation failure rate decreases the further the embryo tends towards the blastocyst stage. Embryos that have reached day 5 and have morphological characteristics of a good blastocyst, have the advantage of showing higher implantation rates. During the first few days of embryo development after fertilization, some embryos do not progress, their cell divisions and growth stop and eventually they do not reach the blastocyst stage.
- Control of multiple pregnancies. The transfer of embryos at the blastocyst stage enables us, due to high implantation rates, to transfer a single embryo, avoiding a multiple pregnancy. However, even in this case there is a very small chance of twin pregnancy due to rupture of the embryonic disc before the 12th day after fertilization, resulting in monozygotic twins which look identical and are of the same sex. A slight increase in the number of monozygotic twins appears to be obtained by embryo transfer of frozen blastocysts.
- Better “uterus-embryo” timing and therefore embryo transfer with a higher chance of implantation. The uterine environment in IVF has much more similarities to the environment in the uterus during normal conception. Likewise, a blastocyst stage embryo transferred is similar to a developing embryo implanted in the uterus during normal conception. Trying to simulate what happens in IVF with what happens in nature also gives us better results.
Embryo transfer on day 3: What are the advantages?
The main advantage of early embryo transfer is that more embryos are obtained. As the days of culturing in the laboratory pass, some embryos are going to stop growing and are no longer viable. For this reason, some experts prefer to transfer them on day 3, when the embryo has 6 to 8 cells. In addition, the number of embryos that can be frozen is larger and can be used for future embryo transfers.
More about the blastocyst
There are couples with many embryos after fertilization, and a large percentage of them develop well. These couples are advised by the scientific team of the IVF center to proceed to embryo transfer of blastocysts. This enables the selection of the most suitable embryos for transfer using finer criteria.
However, as each case is different, it is recommended that you consult the doctor who will perform your IVF and who, in collaboration with the embryologists, will choose the best option for you.
