Pregnancy is a fundamental and evolving process to sustain life in universe. In human, the reproduction is a more complex and highly regulated process. The optimal maturation and the successful fertilization of gamates lead to the successful development of an embryo. Synchronization between embryo development to the blastocyst and differentiation of the endometrium to receptive state is essential to implantation. Greater understanding of sperm and egg development and fertilization is one of the major basics of the clinical application of the assisted reproductive technologies (ART). The abnormalities of these unique processes result in difficult conceiving or infertility. The prevalence of infertility reaches 10~15% of reproductive age couples all over the world, and KIHASA (Korea Institute for Health and Social Affairs) reported in 2003 that it was about 13.5% in Korea. Because most infertile couples, except some specific instances, have difficulties in conceiving, they can be pregnant with infertility treatment such as IVF (in vitro fertilization) or ICSI (intracytoplasmic sperm injection). Generally female fecundability decreases with increasing age. So it is important to try to conceive in earlier ages. Recently, highly developed ART makes it possible to overcome almost all infertility problems.