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The quality of the embryo is an important factor, although not the only one, in the success of an IVF pregnancy.
Once the embryologist has determined that fertilization has occurred, usually between 16 and 18 hours after the eggs and sperm are placed together, it is allowed to develop for another 24 to 36 hours.
Forty-eight hours after fertilization, the clinician evaluates the size and number of the divided cells, as well as the degree of fragmentation, the thickness of the shell surrounding the embryo, and the number of nuclei per cell, among other indicators, to determine potential viability. At this point, there should be at least two evenly sized and shaped cells (four are preferable), with a minimal amount of fragmentation.
In some instances, the embryo may be transferred at the four-cell stage on Day 2, or at the six-cell stage on Day 3. However, some women may have a better chance of becoming pregnant when the embryo becomes a blastocyst, usually on the fifth day of development. The embryologist looks for a single nucleus in each cell, or blastomere. More than one nucleus per blastomere indicates chromosomal abnormalities.
With more regular cells and less fragmentation, the blastocyst has a better chance of adhering to the endometrial lining and developing into a normal fetus. However, there are instances of successful outcomes with embryos that may not appear ideal. Other factors, such as the smoothness of the embryo transfer, are critical to achieving a full-term pregnancy, and it is not easy to predict an outcome solely based on embryo quality.