What are blastocysts and how does the transfer work?

The blastocyst is a stage of the embryo that usually begins on the fifth day (sometimes on the sixth, seventh or eighth day) from the time of fertilization or conception, that is when a male gamete (spermatozoon) and a female gamete (egg) join together to form a single cell called a zygote. 

“The blastocyst is the most advanced stage of fertilization and the one in which the embryo implants. In fact, it is a complex cell structure consisting of many organized cells (more than 200) resulting from a development process that occurs through a series of cell divisions (called cell differentiation),” Dr. Maxia explains. 

The blastocyst has completely different structural features from the embryo of earlier days. In fact, it is composed of specialized cells organized into 3 areas: 

• the outer trophoectoderm, a cell mass that will give rise to the extra embryonic tissues, placenta and amniotic membranes;
• the blastocele, or inner cavity;
• the Inner Cell Mass, a collection of cells within the blastocele that will give rise to the fetus. 

Externally to the blastocyst there is an ever-thinning membrane, called zona pellucida, which when it undergoes lysis (dissolution) allows the trophoblast, i.e. the peripheral cell layer of the egg, to make contact with the inner uterine wall (endometrium) to implant, emitting plasma beta-hcg, hormones that, through analysis on the mother, allow identification of whether pregnancy has occurred.

The embryo is the cellular organism that begins to form after fertilization of the oocyte by the spermatozoon. 

“From this fertilization comes a cell, called a zygote, 2-pronucleus stage that contains the couple's genetic makeup. Within 5 days, the zygote changes with a series of cell divisions, going through different developmental stages necessary for implantation in uterus:

• embryo with 2-4 cells (second day after fertilization) and with 6-8 cells (third day after fertilization);
• morula, embryonic stage with 16-32 cells (fourth day after fertilization), named for its resemblance to a blackberry, during which cells begin to compact;
• blastocyst, the most advanced stage of embryonic development, on the fifth to sixth day of fertilization. The name comes from the so-called “blastula” shape, meaning elliptical or circular.”

In a natural pregnancy, the embryo that has become a blastocyst, usually 5 to 6 days after fertilization, is ready to attach to the endometrium with the so-called implantation process. In the following days, the blastocyst develops and adapts to the conditions of the endometrium to ensure proper nutrition and development. 

Instead, in the context of a medically assisted procreation pathway, blastocyst implantation is carried out through a medical procedure, whereby the embryo is transferred inside the woman's uterus through the insertion of a catheter (embryo transfer). 

“Embryo transfer can be done at the morula stage, but transferring a blastocyst, possibly on day 6, certainly offers many advantages and increases the likelihood of success. In fact, the embryo that reaches this stage has surpassed all previous ones and is particularly resilient, high-performing, and competent. 

In addition, it has been shown that there is a higher pregnancy rate because it is the natural implantation stage with better synchronization between the endometrial stage and the embryonic stage. 

Another advantage is that there is natural selection so that possible aneuploidies or chromosomal abnormalities occur in lower percentage. Finally, this stage is complemented by the possibility of harvesting cells for preimplantation diagnosis (PGT-A) without harming this type of embryonic stage even if they are removed as they are considered “totipotent”, Dr. Maxia concludes.