The blastocyst is the stage of embryonic development in which the embryo reaches the uterine cavity to be implanted (fifth day post fertilization). For the IVF cycles, the embryo is kept five days in the incubator to reach that stage.
It is usually indicated in patients with repeated implantation failures, in cases where there is a risk of hyperstimulation, in single-embryo transfers, embryo selection with more implantation potential, in the PGD, etc.
Pregnancy rates by blastocyst are very similar to those obtained with 3 days embryos (8 cells).
If you get pregnant, consider your pregnancy as a normal one, though it is convenient that you communicate your obstetrician that it has been conceived through in vitro fertilization.