Blastocyst

After fertilization, an embryo divides over several days. By day 5–6 it reaches the blastocyst stage: a hollow sphere with an inner cell mass (ICM) that becomes the fetus and an outer trophectoderm that becomes the placenta. Blastocysts are graded on expansion, ICM quality, and trophectoderm quality (e.g., 5AA).

Most IVF clinics prefer blastocyst transfer over day-3 (cleavage-stage) transfer because the uterus is more in sync with the embryo's developmental stage and blastocysts have a higher implantation rate per transfer. Blastocysts can be biopsied for PGT and vitrified (frozen) for later use.