The term “blastocyst” is a descriptive name to identify the developmental stage of the embryo.A blastocyst is a highly differentiated, highly developed embryo that has grown to the point where it is ready to attach to the uterine wall (implantation).
It is an embryo at an advanced stage of physiologic development when there are two cell types present: one group of cells that form the placenta, and another group of cells that form the fetus. The embryo spontaneously “hatches” from its shell (zonapellucida) at the blastocyst stage of development and is ready to attach to the uterine wall.
Advances by our superb IVF laboratory staff have been able to provide the proper nutrients to grow embryos in the lab to this advanced stage of development. The further developed the embryos, the better your ability to select the most viable embryos and ability to transfer a fewer number of embryos. This will allow us to maintain or raise pregnancy rates while reducing the number of embryos transferred, thereby reducing the most significant complication – multiple pregnancy.
Why Does the Blastocyst Stage Have a Higher Implantation Rate Than Day Two to Four Stage?
Not all fertilized oocytes are normal, and therefore a percentage always exists that are not destined to establish a pregnancy. The majority of such abnormalities are chromosomal. It has been determined that around 25% of eggs are chromosomally abnormal, and that this problem worsens with maternal age. The culmination of this is that many abnormal embryos arrest or stop growing during development. So by culturing embryos to the blastocyst stage, one has already selected against those embryos with little if any developmental potential. Therefore, a blastocyst has a higher probability of developing into a baby after transfer to the patient. Data on the replacement of human blastocysts on day 5 of development indicate implantation rates twice that of cleavage stage embryos.
A second reason for the increased implantation rate of the blastocyst is that the blastocyst is the stage of embryonic development that should reside in the uterus at implantation. As the environment within the fallopian tube and uterus differ, blastocysts are much more suited to survival and development in the uterus. As a result, enhanced or equally successful pregnancy with blastocyst transfer is achieved with fewer embryos and a reduced incidence of multiple births.