After fertilization, the embryo divides over several days: two cells, four, eight — and by day 5 or 6 it reorganizes into a blastocyst, a fluid-filled sphere 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 a natural conception, the embryo reaches the uterus at around this stage — so transferring a blastocyst matches the embryo to the uterine lining's natural timing. Culturing to day 5 also lets the lab observe which embryos develop most strongly, helping select the one with the best potential. Not every embryo reaches blastocyst, which is informative in itself.
Grading
Labs grade blastocysts on expansion and the appearance of the two cell groups (grades like "4AA" on reports). Grading guides selection, but it's an assessment of appearance — good-grade embryos can fail and lower-grade ones can succeed.