The bottom part of the uterus that’s found in the back of the vagina. It can be as small as a cherry in a woman who has not delivered a baby through her vagina, or it can be much larger. Mucous passes through the cervical opening (os) and bathes the vagina. So does period flow. Sperm that’s in the vagina enters the uterus through the os of the cervix. The cervix feels softer during ovulation and its secretions change at that time, which is an important indicator for couples using natural birth control.