At fertilization check, the embryologist usually sees that some of the eggs have no fertilization.

Eggs fail to fertilize for a variety of reasons.

The embryologist may see that some of the eggs are immature.

As eggs mature, they go through stages of chromosomal division. These stages result in a reduction in their chromosome number by one half. The extra chromosomes are contained in a small, round body located on the side of the egg. This is called a polar body. If an egg does not have a polar body at the time of the fertilization check it is still immature and not ready for a sperm to fertilize it. Also, if it has not formed a polar body by this time (twenty four hours after the egg retrieval), it is defective and will not be capable of fertilization at a later time.

There are some eggs that have a polar body present but are still not fertilized.

They are mature, but have not shown the characteristic formation of their pronucleus or the formation of the sperm’s pronucleus. This can happen if the egg is defective in its number of chromosomes and does not have a complete plan for building an embryo. This results in the egg being incapable of further development. The sperm can also be the cause of a mature egg not fertilizing. The semen sample may have a low number of sperm that are capable of penetrating the egg’s membranes. This is often associated with a high percentage of abnormally shaped sperm. In severe cases, this may result in total failure of fertilization. If this is suspected, intra-cytoplasmic sperm injection can be used to overcome this penetration problem. If the problem with the sperm is chromosomal in nature, fertilization can fail because the sperm does not have a complete plan for building an embryo. In this case, it will not matter if the sperm penetrates the egg, because it cannot form a pronucleus. The egg will not fertilize and it will prevent any further development. Unfortunately in these cases, intra-cytoplasmic sperm injection cannot overcome the problem.

There are some eggs that are in the ovaries that have no chance of developing into an embryo because they are atretic. Atresia is a form of abnormal development in which the cytoplasm, or cell material, of the egg is dark and non-functional. Just as there are sperm that have abnormal morphology and are not functional, so too there are eggs that are abnormal from the moment of their development and have no potential for growth.

Some eggs have perfectly healthy chromosomes and cytoplasm but may have a poorly formed shell, or zona pellucida.

These eggs often break during the aspiration process that is necessary to remove them from their follicle. The result is an egg that loses its shell or one in which the cell material, or cytoplasm, also breaks. Without the protective shell at this early stage, the egg cannot survive and grow normally. If these eggs with weakened shells had not been aspirated, but had been allowed to ovulate and enter the Fallopian tube, the result would likely be the same. They would not have survived the fertilization process in the body and would not have developed a normal embryo without the protection of an intact, strong shell.