Physiology, Cervical Dilation

The cervix is a narrow, tubular passage that comprises the inferior end of the uterus. The uterus is a pear-shaped organ with the narrower part, the cervix, facing downwards and protruding slightly into the vagina in pre-menopausal women. As such, the cervix serves as a passageway between the endometrial cavity and the vagina and is physiologically important in reproduction and childbirth.The cervix divides into the endocervix, the endocervical canal, and the ectocervix. The endocervix is the portion of the cervix that meets the corpus of the uterus. The ectocervix is the most inferior portion of the cervix that protrudes into the vagina. The internal os of the cervix is the opening of the cervix that joins the uterus, while the external os of the cervix is the junction between the cervix and the vagina. During labor, the internal os will undergo effacement, while the external os will undergo dilation. The cervix serves as a barrier between the endometrial cavity and the vagina, the latter essentially being the external world. Depending on the phase a female is in during her menstrual cycle, the cervix will produce mucus of different consistencies to either facilitate or hinder sperm entry into the endometrial cavity. Furthermore, during pregnancy, the cervix produces a thick mucus plug that keeps pathogens out of the endometrial cavity to protect the developing fetus and to maintain a sterile environment in the uterus.Towards the end of pregnancy, as the time of labor approaches, the cervix must thin and open wider to facilitate and accommodate the passage of the fetus during delivery. This process is known as cervical effacement and dilation. The determining factor for the first stage of labor is cervical progression; when the cervix has become completely effaced and dilated, it marks the beginning of the second stage of labor. Should this process become impaired or proceed too slowly, the obstetrician can augment cervical effacement and dilation.