"Occasionally, fertilization with conventional IVF is unsuccessful and no eggs fertilize normally. There may be complete fertilization failure or abnormal fertilization (i.e. polyspermic fertilization where more than one sperm fertilizes an egg). Egg and/or sperm defects can cause fertilization failure. If a significant percentage of eggs fail to fertilize (i.e. there are too few fertilized eggs for a reasonable chance of pregnancy), IC.SI may be performed on the unfertilized eggs the day after egg retrieval. There are only a few reports of pregnancies from eggs fertilized by ICSI on the day after retrieval. Immature eggs cannot be fertilized unless they complete the maturation process in culture. Some immature eggs require up to 24-36 hours in culture to mature. It is not unusual to observe “late” fertilization of such eggs up to 24 hours after insemination. However, the chance of a successful pregnancy from these late fertilized eggs is slim."
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