Once fertilization has occurred and normal embryo development is confirmed, it is transferred to the uterus. If implantation occurs, the remaining development of the embryo occurs naturally within the mother. Since replacement of up to four embryos maximizes the chances of success while minimizing the risk of multiple gestation, it may be advantageous to store excess embryos through cryopreservation. If conception does not result from the treatment cycle, the stored embryos can be thawed and transferred to the patient during a natural cycle without her having to undergo ovulation induction and egg retrieval. ICSI has revolutionized the treatment of male infertility by overcoming barriers to sperm entry into the egg.
Fertilization is possible even in cases with scant sperm or compromised sperm function.
Through the increased number of swimming, and presumably functional, sperm around the egg, the chances of fertilization are improved over intercourse or intrauterine insemination. In addition, there are men who only have immature sperm available, and these may not be fully functional. An ideal approach to these severe forms of male infertility is to directly inject a single sperm cell into the egg. In this way, the sperm cell is relieved of the work of penetrating the several layers which surround, nourish and protect the egg. It is also apparent that relatively few sperm are needed since only one sperm cell is required for each egg.