In a standard IVF cycle embryos are transferred to the womb two to three days after egg collection and insemination (this is referred to as a Day 2 or 3 transfer).
With a blastocyst transfer, the embryo has advanced to the five or 6-day stage. This means the embryo has divided many more times into many more cells over this period. Blastocysts have a very thin outer shell thus potentially increasing the chances of implantation into the uterine cavity. While the majority of fertilised eggs will develop into a three-day old embryo, only perhaps 40% of these embryos will develop into a blastocyst, so they are considered to be a more "select" group of embryos with a higher chance of pregnancy.
Research has shown that conditions in the womb may be more optimal for a blastocyst than a day 2/3 embryo as there are slightly differing conditions in the fallopian tube and the womb.
Data suggests that blastocyst transfer can increase the chances of a live birth.
Some patients have a high number of frozen embryos and one option is to thaw all embryos and culture through to the blastocyst stage to allow the best 1 or 2 embryos to be replaced based on their development. Please remember that the extended culture process doesn't enhance an embryos quality per se, it is a way of choosing the most viable from a group of embryos.