What is Blastocyst?
A blastocyst is a highly developed embryo that has divided many times to a point where it is nearly ready to implant on the walls of the uterus. A blastocyst has come a long way from its beginning as a single cell.
During maturation, an embryo rests inside a protective shell called a zona pellucida. You can think of this protective shell as being much like a chicken egg. But, unlike chicken eggs, human embryos do not remain inside a shell. Instead, the embryo hatches (breaks out of the shell) on the fifth or sixth day so it can attach to the uterine wall (implantation). Just prior to hatching, an embryo becomes a blastocyst.
Embryos developing to the critical blastocyst stage have a much greater chance of implanting successfully and resulting in an ongoing pregnancy. That is because these embryos have passed an important test. During the first few days, the embryo relies on the mother's egg for all its nutrients. However, in order to 15 survive past day three or four, the embryo must activate its own genes. Not all embryos are successful. In fact, only about one-third of the embryos become blastocysts. Yet these embryos are more highly-developed, healthier, and stronger, and have a higher rate of implantation when compared to day three embryos. Due to the higher probability of survival, we transfer fewer back into the uterus.