Under natural circumstances, the union of sperm and egg is a long marathon. Most of the sperm that are ejaculated into the vagina are expelled with the semen, while those that are lucky enough to swim through the vagina are blocked by the cervical mucus and engulfed by the white blood cells in the uterine cavity. A small percentage of the sperm that are both strong and lucky enough to “make it” will continue to swim to the fallopian tube where they will wait to join the egg. The survival time of the sperm in the fallopian tube is 1-3 days. After the egg is expelled, it is captured by the umbrella of the fallopian tube and stays in the abdomen for about 1 day. The probability of successful union of sperm and egg is greatly increased when intercourse takes place around the woman’s ovulation day. In the embryo lab, the process of sperm-egg union is much simpler. The washed and upstream-selected sperm are placed directly around the egg, and they only need to release the acrosome enzyme in their heads to disperse the granulosa cells wrapped around the egg to see the egg in its original form. However, it is not easy to break up the thick layer of granulosa cells, as the acrosome enzyme of one sperm is not enough and many sperm need to follow to create a path to the egg. Eventually, the first sperm to reach the zona pellucida and not yet depleted of energy will cross the zona pellucida and enter the egg to complete fertilization. Once one sperm enters, the egg zona pellucida and cell membrane undergo a series of changes that prevent other sperm from entering.