A blastocyst is an embryo that has been developed in laboratory for 5-6 days after insemination, in contrast to IVF which includes transferring embryos to uterus after egg collection and insemination. There are five to six stages in blastocyst transfer. This implies that embryo is partitioned many more times into many more cells over this period.
Blastocyst has thin outer layer shell which increases the chance of implantation into uterine cavity. The greater part of the blastocyst contains a liquid pit and it is possible to see the cells which will end up being the baby and those which will make up the placenta.The majority of the fertilized egg is developed into three day embryo and later in blastocyst.
So blastocyst is considered to have great chance of pregnancy.The 5 or 6 day blastocyst is advanced structure than the 3 day old embryo and the real advantage of transferring blastocysts is the high live birth rate associated with blastocyst transfer.