Insemination procedures entail bringing together the male and female gametes – sperm and eggs – in order to achieve fertilization. These procedures are performed approximately 40 hours post-hCG (4-6 hours post retrieval) in the IVF laboratory. They include standard in vitro fertilization (IVF) and a more advanced, microsurgical procedure called intracytoplasmic sperm injection (ICSI). During standard IVF, large numbers of sperm (approximately 10,000 per medium droplet containing oocytes) are added and the gametes are co-cultured in the incubator to allow for fertilization. During ICSI, a single sperm is microsurgically injected into the oocyte and the injected oocyte is placed in culture for several hours to allow for the completion of the fertilization process. ICSI requires some additional steps as the follicle cells surrounding the oocyte must be removed and nuclear maturity must be assessed before the actual injection. There are no differences between fertilization rates achieved by IVF and ICSI procedures. ICSI is generally the method of choice when a male factor is present and sperm parameters are not optimal. ICSI is also used in case of previous fertilization failure or more than 2 years of unexplained infertility. In all cases, fertilization is assessed approximately 18 hours post-insemination. Oocytes are considered to be normally fertilized if they contain 2 pronuclei (2pn). All zygotes with two pronuclei are placed into new culture dishes and placed back into the incubator for further growth and development.