http://www.infertilitydoctor.com/fertilityflash/vol3_issue5.htm
During an egg retrieval procedure, an embryologist, with the aid of a microscope, quickly identifies and takes custody of the eggs. They are immediately placed inside an incubator in our laboratory where they mature for 4 hours before being introduced to sperm. Then, depending on the quality of the sperm, an embryologist will either incubate each egg in the presence of 100,000 sperm, or inject a single sperm into each egg (in a process called Intracytoplasmic Sperm Injection or ICSI).
The next morning, 22 hours after the egg retrieval, an embryologist observes the eggs and records their fertilization status. A small percentage of eggs will be immature or non-viable. The remainder can fertilize normally, fail to fertilize or fertilize abnormally.
Figure 5 is relevant to myself -
While performing fertilization checks, the embryologist will observe either no pronuclei or 2 pronuclei in the vast majority of eggs. However, a small number of eggs, usually around 5% of all eggs inseminated, will show some number other than none or two pronuclei (see Figures 3, 4 and 5). These are the eggs that are abnormally fertilized.
Having more than 2 pronuclei usually suggests that more than 1 sperm managed to enter the egg. If this happens, there is one nucleus from the egg and one nucleus for each sperm that got in. An egg penetrated by 3 sperm for example, will have 4 pronuclei (see Figure 5). Please note that the egg has 3 separate mechanisms to prevent penetration by more than one sperm, so eggs with >2 pronuclei are uncommon.
This is an abnormally fertilized egg with four pronuclei (circled in blue).
