Depending on how many embryos are produced and their quality, embryos can be left in the lab for 5–6 days to become a blastocyst after fertilization.
It possesses an inner cell mass (ICM), and an outer layer of cells surrounding the inner cell mass and a fluid-filled cavity known as the blastocoele. The human blastocyst comprises 70-100 cells. An enzyme of the cells on the exterior, erodes outer lining and creates a site for implantation. (There by increased chances of success rate).
Pros and cons of Blastocyst Transfer
Under the standard IVF culture conditions, only about 50% of human embryos progress to the Blastocyst stage after 5 days of culture. The low rates are either due to the inherent “weakness” of human embryos or inexperienced labs with Blastocysts.
Thus, it is not really advised in cases where we have less number of embryos, or higher age of the eggs (mother). Keeping embryos for Blastocyst may also mean fewer embryos left to freeze.
Research proves, Blastocyst transfer results in a higher success rate of becoming pregnant as compared with day 2 / 3 embryo transfer, as that is the time for actual embryo implantation inside the uterus.