Sperm remain inside the vagina and uterus due to several biological mechanisms. When semen is transferred to the vagina, sperm activate and begin swimming towards the egg. The pH and temperature of the vagina are favorable for sperm movement and survival. Additionally, the cervical mucus becomes thinner and clearer during ovulation, which facilitates the passage of sperm towards the egg. Sperm can remain viable in the female reproductive system for up to 5 days, increasing the possibility of fertilization.