While you're at home recovering from the retrieval, the follicles that were aspirated will be searched for oocytes, or eggs. Not every follicle will contain an oocyte.
Once the oocytes are found, they'll be evaluated by the embryologist. If the eggs are overly mature, fertilization may not be successful. If they are not mature enough, the embryology lab may be able to stimulate them to maturity in the lab.
Fertilization of the oocytes must happen with 12 to 24 hours. Your partner will likely provide a semen sample the same morning you have the retrieval. The stress of the day can make it difficult for some, and so just in case, your partner may provide a semen sample for backup earlier in the cycle, which can be frozen until the day of the retrieval.
Once the semen sample is ready, it'll be put through a special washing process, which separates the sperm from the other stuff that is found in semen. The embryologist will choose the “best looking sperm," placing about 10,000 sperm in each culture dish with an oocyte. The culture dishes are kept in a special incubator, and after 12 to 24 hours, they are inspected for signs of fertilization.
With the exception of severe male infertility, 70% of the oocytes will become fertilized.
In the case of severe male infertility, ICSI (pronounced ick-see) may be used to fertilize the eggs, instead of simply placing them in a culture dish. With ICSI, the embryologist will choose a healthy-looking sperm and inseminate the oocyte with the sperm using a special thin needle.