Under natural conditions, the female and male gametes meet and fertilization takes place in the Fallopian tube. As it can be seen in Figure 5, the embryo travels for 3 days through the tube until it reaches the uterine cavity, where it hatches, that is, it leaves the zona pellucida or outer membrane covering the egg. If development is normal, the embryo implants in the fifth or sixth development day, in a stage called blastocyst.
Human embryo development seen with time-lapse during the first 5 development days. From two cells until hatching (fifth to sixth day), corresponding to the blastocyst leaving its external coat, the so-called Zona Pellucida, to implant in the endometrium, and in this way, starting pregnancy.
Figure 5. Schematic image of the Fallopian tube, where fertilization occurs under spontaneous conditions.
As seen in Figure 6, no more than half of the fertilized eggs (in the lab or in the Fallopian tube) have the capacity to reach the blastocyst stage.
Figure 6. Spontaneous embryo mortality throughout the pre-implantation development.
The probability of achieving a pregnancy is higher if blastocysts are transferred, compared with developing embryos (days 2 to 4), which can die spontaneously before reaching blastocyst stage; however, not all women are able to produce embryos that reach this stage.
As it can be seen in Figure 7, if a woman is transferred with one or two blastocysts chosen on the fifth day, the chances of pregnancy are always higher than if she is transferred with embryos chosen on the third day of development.
This figure also teaches us that two blastocysts must not be transferred together, because the possibility of twins is 30 times higher than if only one is transferred.
The lab conditions required to have embryos during 5 days are more demanding than those to have embryos for 2 or 3 days. Therefore, it is important for each center to decide which is best according to their own conditions.
Figure 7. Delivery rate transferring embryos at days 3 and 5, Latin America, 2020.
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.
