Embryo development is a crucial phase that occurs after fertilization but before implantation in both IVF and ICSI procedures. It all begins with the formation of a zygote, typically within 30 hours after the union of pronuclei. The precise timing of this step is critical for healthy embryo development, ideally occurring within this 30-hour window.
The subsequent stage involves cell division at specific intervals. By the 48-hour mark, which is the 2nd day following egg retrieval, healthy embryos should ideally have divided into 2 to 4 cells. By the 72-hour point, marking the 3rd day of development, it is expected that embryos will have reached at least 6 cells. These well-timed cell divisions are indicative of robust embryo development and are significant indicators of the embryo’s potential for successful implantation and further development.
Embryo Development Stages
Embryo development in the context of IVF progresses through three distinct stages before the crucial process of implantation.
The initial phase, termed the “Cleavage” stage, spans a duration of approximately three days. During this period, meticulous observation is employed to monitor the number of cell divisions and the degree of fragmentation. Minimizing fragmentation is imperative for optimal embryo development. An excess of fragmentation, exceeding 25%, may diminish the likelihood of successful implantation.
The count of cell divisions is another pivotal parameter for ensuring healthy embryo development. By the conclusion of the third day, the embryos are ideally composed of a minimum of six, and preferably eight, distinct cells. These individual cells, within cleavage stage embryos, are scientifically referred to as blastomeres.
Following the Cleavage stage, the embryo advances to the Morula stage on the fourth day. At this juncture, the embryo is comprised of approximately 16 cells. While cell division continues, it becomes progressively challenging to ascertain the precise number of cells. This transition marks a significant milestone in the embryo’s journey, bringing it one step closer to potential implantation and further maturation.
In the second stage of embryo development, a process known as “compaction” takes place. During this phase, the blastomeres form tight junctions with one another, transforming their round shape into a mulberry-like form. This distinctive mulberry shape gives the embryo the name “morula.”
The third and final stage before implantation is referred to as the “Blastocyst” stage. It typically occurs by the fifth or sixth day following fertilization. During the Blastocyst stage, the formation of junctional complexes between blastomeres creates a rapidly dividing ball of cells, establishing an inner and outer construction. The inner group of cells will eventually become the embryo, while the outer group will serve as protective and nourishing cells, signaling the embryo’s readiness for implantation.
After the Blastocyst stage, if all progresses as expected, the individual will proceed with the implantation process. Embryo development after transfer continues, and this journey spans a period of nine days. After this timeframe, it becomes possible to detect the individual’s pregnancy.
Now, let’s delve into the day-by-day changes in the embryo during the crucial period before implantation after transfer:
1st Day after embryo transfer: The blastocyst initiates the hatching process, gradually breaking free from its outer shell. Completing this hatch-out procedure will take an additional two days.
2nd Day after embryo transfer: As the blastocyst finishes hatching, it starts to attach itself to the uterine lining, a pivotal step in the implantation process.
3rd Day after embryo transfer: Implantation officially commences as the blastocyst attaches more deeply into the uterine lining, a process that continues through the end of the fourth day.
5th Day after embryo transfer: The implantation of the blastocyst is now complete, initiating the phase where cells begin to transform into the placenta and fetus.
6th Day after embryo transfer: With the successful completion of implantation, the human chorionic gonadotropin (hCG) hormone enters the individual’s bloodstream. It will take an additional two days for hCG levels to reach the ideal threshold for pregnancy detection through blood tests.
9th Day after embryo transfer: By this point, hCG levels have risen sufficiently to confirm pregnancy through a blood test.
