The process of union of sperm and egg is called fertilization, which mainly involves the entry of semen into the woman’s vagina and the sperm reaching the juxtaposition of the fallopian tubes, where it combines with the egg to form a fertilized egg. The sperm-egg union is a very complex physico-chemical process, in which the sperm is energized by contact with the outer membrane of the egg, and is reflected through the acrosome as it passes through the zona pellucida and the radial crown of the egg, and ultimately enters the oocyte and fuses with the oocyte to form a fertilized egg. The process of sperm-egg union takes about 24 hours in most cases, but it is not possible to determine the exact time. Once the sperm and egg have united, a fertilized egg is formed, which then begins to lay and develop in the uterus.