A blastocyst 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. 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.
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). In naturally conceived pregnancies, the egg is released from the ovarian follicle and picked up the fallopian tube where it is fertilized by sperm. The resulting embryo starts out as a single cell, which then must grow and differentiate until it has the capacity to attach to the uterine wall. The embryo divides from one cell into two cells, then four cells, eight cells, 16 cells, etc. until it reaches several hundred cells at the time of implantation and reaches the blastocyst stage on day five or six after ovulation. The term “blastocyst” is a descriptive name to identify the developmental stage of the embryo.
The embryo spontaneously “hatches” from its shell (zona pellucida) at the blastocyst stage of development and is ready to attach to the uterine wall.
A blastocyst transfer is a technique, incorporated with in vitro fertilization (IVF), designed to increase pregnancy rates and decrease the risk of multiple pregnancy. This technology, pioneered by investigators from Australia has been reported to increase the pregnancy rate in selected patients by almost double, while virtually eliminating the risk of high order multiple pregnancies, like triplets or quadruplets.
Approximately one third of embryos are capable of successfully activating their genes and growing to the blastocyst stage. If an embryo reaches the blastocyst stage, nature is telling us that it is a healthier embryo and it has a better chance of implanting successfully and resulting in a normal, healthy, baby.
In conventional IVF, embryos are grown for only 2-3 days before they are replaced into the uterus. The embryos have between 4-8 cells inside the shell (zona pellucida) and must continue their growth and development inside the uterus for 4-5 more days before they become blastocyst embryos and are ready to implant. If a couple produce many embryos in an IVF cycle, the embryologist, no matter how skilled, cannot really tell which embryos have the capacity to grow into a blastocyst embryo and which do not. By selecting a group of embryos for uterine replacement (embryo transfer) on day two or three after ovulation, it is too early to be certain which embryos are capable of blastocyst development. This is why conventional IVF is usually done by transferring 2-3 embryos. No one hopes that all of these embryos are going to implant, but recognize that maybe only one or two have the ability to form blastocysts and succeed in developing into a normal pregnancy. Keeping the embryos in the IVF laboratory for another 2-3 days allows blastocyst development and allows you and the IVF team to identify the best embryos for replacement. Now this is where the good news comes!
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 or 6 of development indicate implantation rates twice that of cleavage stage embryos have been reported.
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.