Cervical mucus is a viscous substance produced in the cervix. The cervical mucus is the protective barrier of the uterus: it is indeed what stops the intrusion of germs that can be harmful to the health. The cervical mucus changes at certain times of the cycle to promote the passage of sperm. It creates a favorable environment for them to survive and move towards the egg in order to maximize the chances of fertilization. During the non-fertile period, the mucus changes and seals their passage.