Morphological assessment of embryos is evaluated on days 0, 1, 2, or 3. Assessment on days 5 and 6 (or occasionally on day 4) is associated with extended culture to the blastocyst stage. Day 0 evaluation is based on oocyte morphology and only used when intracytoplasmic sperm injection (ICSI) is performed when cumulus and corona cells are removed. A normal zygote forms on day 1 and should have two pro-nuclei (PN) that are of equal size, closed, and in the center of the cytoplasm; possess at least three nucleolar precursor bodies (NPBs) in each PN; and NPB that are polarized (preferentially) or nonpolarized in both PN. Transferring embryos at the blastocyst stage has become an option for most assisted reproductive clinics due to the development of sequential culture media. Different monitoring systems allow embryologists to visualize the changes that occur while in culture, such as cleavage, fragmentation, and blastulation.