In order to survive past the first three to four days, the fertilized embryo must grow to become a blastocyst. Only about one-third of the embryos are successful in this regard. These embryos, now considered blastocysts, are more highly-developed, healthier, and stronger, have a higher rate of implantation when compared to younger embryos, and in general are more adapted to survive in uterus. The embryo reaches blastocyst stage at around day 5-6 after fertilization.
During maturation process, the embryo is encased inside a protective shell. This protective shell, also known as zona pellucida, much like a chicken egg, protects the embryo from outside influence for around 5 to 6 days. On the 5th or 6th day, the embryo hatches and implants itself to the uterine wall.
At that moment, the embryo contains around 200 cells, making its implantation surface larger. Because of that, a blastocyst, once in uterus, has a higher chance to implant itself than a smaller, less developed embryo. Statistics show that a succesful blastocyst implantation increases the possibility of succesful pregnancy by around 50%.
Blastocyst cultivation in laboratory conditions allows the gynecologists to select the healthiest and liveliest embryos to be transfered into the patient's uterus. This, in turn, increases the possibility of a succesful ongoing pregnancy and severely reduces the risk of multiple pregnancy. Blastocyst implantation procedure is performed if earlier fertility treatments are proven unsuccesful.