Embryo cryopreservation/freezing technology has significantly changed and improved during the last 30 years. The currently used technology, called vitrification, as it name suggests, aims at rapidly reaching very low temperatures (-196°), therefore stopping the biological processes very fast, but maintaining cell properties. So, the frozen tissue, in this case the embryo, can remain unaltered, maintaining its properties when thawed, after decades.
It is difficult to know exactly for how many decades embryos can be kept unaltered, but surely for 20 years or more. In general, embryo freezing is performed in blastocyst stage (5 to 6 development days). Even though embryo survival at thawing highly depends on the quality of the vitrified blastocyst, vitrified blastocyst survival usually varies between 95 and 98%. Moreover, as we will see later, the probability of achieving a pregnancy and a delivery after transferring fresh and thawed blastocysts is practically the same.
As it can be seen in Figure 11, the chance of birth decrease with the age of woman, and this is mostly related to the quality of the embryos resulting from chromosomal abnormalities. The purple column shows the possibility of having a livebirth after transferring a fresh embryo, and the orange column shows the possibility of having a lifebirth after transferring the fresh embryo plus a frozen one if pregnancy was not achieved with the fresh embryo transfer. This is called cumulative probability of live birth.
Figure 11. Delivery rate after fresh embryo transfer and cumulative delivery rate when transferring Blastocysts according to woman’s age.
Outcomes of frozen embryo transfers (FET) have been changing throughout the years. As it can be seen in Figure 12, in the 90s and until 2012, chances of pregnancy were better with fresh than with frozen embryos. However, technology has significantly improved and, currently, freezing does not seem to affect embryo survival. The chance of having a baby after frozen embryo transfer is higher than that by fresh embryo transfer.
Figure 12. Delivery rate by fresh and frozen/thawed embryo transfers.
Fresh and frozen blastocyst transfers have the same chance of a live birth. Moreover, on some occasions, a frozen embryo transfer is preferred, in order to avoid possible abnormalities in endometrial development, which may result from the hormonal stimulation process.
It is worth recalling that when the ovaries are overstimulated, the resulting amount of hormone in the woman triples that of a natural cycle. This estrogen excess (female hormone) may, on occasions, negatively affect the endometrium and affect embryo implantation. For this reason, in certain cases it is preferred to freeze the embryos and transfer them in a delayed cycle when the endometrium is in balance with the natural cycle.
Figure 13. Proportion of frozen embryo transfers (FET). RLA 1996-2020.
As seen in Figure 13, most embryo transfers are currently performed with frozen/thawed embryos (66.6%). Embryo freezing allows to: 1) perform genetic/chromosomal studies in the embryos, 2) achieve more than one embryo transfer from one follicular aspiration (Cumulative Pregnancy and Delivery Rate), and 3) better prepare the endometrium in cases of endometrial abnormality. Furthermore, embryo freezing makes it possible to reduce the risk of severe ovarian hyperstimulation, mainly in women with polycystic ovaries. 4) This technology has made the largest contribution reducing the number of embryos transferred, thus reducing multiple gestation rates (twins, triplet and more).
The Latin American Registry of Assisted Reproduction (RLA) thanks the 204 institutions accredited by REDLARA which, every year, report their data to RLA.
This voluntary and rigorous contribution makes it possible to generate a powerful database, which helps doctors and patients take decisions based on the best existing regional evidence.
This educational portal is the result of a collaborative activity between the Latin American Network of Assisted Reproduction (REDLARA), and University Diego Portales from Chile, which allow the link between the reproductive science and technology advances with the needs of people suffering from infertility and other forms of reproductive impairments.
We thank Dr. Ariel Ahumanda and Dr. Gustavo Martínez from Argentina for: images and videos of spontaneous in vitro fertilization, ICSI and PGT,
Dr. Roger Molinas from Paraguay for: videos of follicular aspiration and embryo transfer,
Dr. Javier Crosby from Chile for: images of different stages of in vitro embryo development,
Dr. Yasuyuki Mio from Japan for his video of in vitro embryo development.
We especially thank the industry for providing the financial support, which is so necessary to keep the Latin American registry as a pillar of this international cooperation, which today includes 16 countries and more than 200 collaborative institutions, from Mexico in the north to Chile in the far south.
