A blastocyst is a cellular mass that forms early in the process of embryo development in mammals. In humans, this stage of embryogenesis occurs five days after fertilization, when there are fewer than 100 cells in the mass. At this stage of development, the embryo has not yet implanted in the uterus.
Fertilized eggs are known as zygotes. Once fertilized, the egg undergoes a rapid series of cell divisions, followed by differentiation of the cells.
Within the blastocyst, there are two types of cells. In the interior is the inner cell mass, a portion of which will begin to divide at a very rapid pace and become the developing fetus. It is surrounded by a fluid-filled cavity called the blastocoele. Surrounding this is a single layer of cells called the trophoblast, which will combine with the endometrial cells of the uterus to become the placenta.
Many people undergoing in vitro fertilization prefer blastocyst transfer as an alternative to three-day embryo transfer. Transferring blastocysts, which are slightly older, can be a more effective option because they tend to be more stable than three-day embryos and more likely to result in a pregnancy. This is because approximately 50% of embryos do not reach day five of development; by using these embryos, therefore, there is a greater chance that they will be viable after transfer.
Another advantage of using these older embryos for implantation is that, because there is a greater chance of viability, not as many need to be implanted. Many people prefer this option because there is a much lower chance of multiple pregnancies, which can be dangerous for the pregnant woman and for the developing babies, with an increased risk of underweight babies, miscarriage, and still birth.
Blastocysts are important in a field of scientific research called embryonic stem cell research. Embryonic stem cells are believed to have special properties that other types of stem cells do not have, perhaps due to the fact that, in embryonic cells, gene expression has barely begun. Embryos used in this type of research are obtained from in vitro fertilization clinics with the express permission of the biological parents. Generally, people allow their embryos to be used in this way if they are no longer needed for implantation, and if they are comfortable with them being used in research.