Blastocyst culture is the incubation of the embryo until five days after the egg retrieval. By day 5, the embryo has divided into two cell types: inner cell mass (ICM) which becomes the baby, and the trophectoderm cells which will become the placenta.
Only a small percentage of embryos will ultimately reach the blastocyst stage (about 20% to 25% of genetically normal fertilized embryos). The first few days after fertilization, the embryo relies on the maternal genes, but by day three or day four, the embryo must activate its own genes.
This becomes “survival of the fittest” and helps the embryologist choose which embryos have the highest chance for implantation. The blastocyst culture is preformed by the embryologist as a type of self-selection.
Embryos which develop to the blastocyst stage have some of the best probability for producing a pregnancy. Blastocyst transfer also helps reduce multiples because we can transfer less embryos.