After fertilization, the embryo divides over several days: two cells, four, eight. By day 5 or 6 it becomes a blastocyst. This is a fluid-filled ball with two distinct parts: an inner cell mass that becomes the baby, and an outer layer that becomes the placenta.
Why the stage matters in IVF
In natural conception, the embryo reaches the uterus at about this stage. So transferring a blastocyst matches the timing the uterine lining expects. Growing embryos in the lab to day 5 also shows which ones develop most strongly, which helps pick the best one to transfer. Not every embryo reaches the blastocyst stage, and that itself tells the lab something useful.
Grading
Labs grade blastocysts on expansion and the appearance of the two cell groups (grades like "4AA" on reports). Grading helps choose which embryo to transfer, but it only judges how the embryo looks. Good-grade embryos can fail, and lower-grade ones can succeed.