Cervix is the muscular opening of the uterus. With the approach of ovulation it becomes softer, opens, and raises its position relative to the uterus. At infertile times it is low, firm and closed. The opening of the uterus is a thick muscle known as the Cervix. As ovulation approaches, the same hormones that cause cervical fluid to be secreted also cause changes in the position, texture, and opening of the cervix. During infertile periods (Phase 1 and 3), the cervix is low, firm and closed (during Phase 3, it is closed tightly). It is so low that it is fairly easy to reach for observation purposes. As fertility increases, the cervix rises in position, becomes softer, and starts to open. At peak fertility, the opening of the cervix is about 15 mm wide.

These changes make it easier for sperm to reach the ovum -- conception wouldn't take place if the Cervix is not open. If the cervix didn't close tightly after ovulation, the uterus and fertilized egg could become infected. The cervix has to open somewhat at menstruation to allow the endometrium to be shed. Its use in fertility awareness is primarily as an additional sign when temperature and cervical fluid do not define by themselves when post-ovulation infertility has begun.