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.[1]

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, which is effectively 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.[2] 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.

Dilation of the cervix is achieved when the cervix dilates to 10 cm. How rapidly a female will progress to full dilation is dependent on several factors, including her parity, medical history, her pelvic anatomy, the size of the fetus, and the position of the fetus at the time of labor.[3][4][5] Multigravidas progress to full dilation more quickly than primigravidas. A history of diabetes mellitus or gestational diabetes increases the likelihood of a large-for-gestational-age fetus and macrosomia, which can lead to labor complications such as the arrest of descent, a prolonged latent phase or active phase arrest.[6][7][8] If the pelvic anatomy of the mother is too narrow or has any other anatomic anomaly, cephalopelvic disproportion between the fetus and the mother may lead to prolonged or arrest of labor.[9] If the fetus is in the breech position, this may also lead to labor abnormalities and hinder the ability of the cervix to dilate fully.[10]

The stages of labor are as follows: Stage 1 is effacement and dilation of the cervix, Stage 2 is the delivery of the fetus, and Stage 3 is the delivery of the placenta. Stage 1 subdivides into latent and active where 0 to 6 cm is the latent phase of Stage 1, and 6 cm to complete dilation of the cervix is recognized as the active phase of Stage 1 labor.[11]

In a primigravida, the latent phase should not exceed 20 hours. In a multigravida, the latent phase should not exceed 14 hours. If a female is in labor and exceeds these average time frames, she is said to be having a prolonged latent phase. Newer data suggest that these parameters are longer.[12] The active phase of labor should occur much more rapidly, usually within 4 hours after the rupture of membranes when there are adequate uterine contractions. If the mother does not fully dilate by these average estimations, she is said to be having an arrest of the active phase of Stage 1 labor. 

The cervix is composed of multiple cell types. The endocervix is composed of columnar epithelium interspersed with mucus-producing goblet cells. Towards the outer edge of the cervix, the epithelial cell type transitions to squamous cell epithelium.[13][14] The ectocervix and the upper third of the vagina are composed of squamous epithelium.[14] The area of transition between the two types of epithelium is called the "transition zone." The transition zone is clinically significant in the development of cervical cancer, which this article discusses elsewhere. 

On a more micromolecular level, the cervix is a collagenous structure composed of proteoglycans, fibrin, and glycosaminoglycans (GAGs). It is not a muscle like a uterus, but instead, it is more elastic and has fibers interwoven into the foundation that gives it the ability to dilate and efface during pregnancy and recoil after delivery, which makes it more similar to connective tissue.[15] The composition of collagen and GAGs enable the cervix to quickly metabolize these proteins and transform from a firm, tubular structure to a soft, flattened structure well suited for birth.[16][15] 

While the mechanism is not entirely understood, studies suggest that the cervix undergoes temporary hyperplasia with a multitude of collagenous cells to support the birthing process.[17] Once delivery is complete, these cells undergo apoptosis and return the cervix to its pre-pregnancy state.[13][18][2]

The cervix and upper one-third of the vagina form from two paramesonephric (Mullerian) ducts that fuse and recanalize, which also gives rise to the body of the uterus and the fallopian tubes. This process occurs during the sixth week of embryological development. The paramesonephric ducts are present in all embryos from day 0. Without the Mullerian inhibiting factor (MIF), which is a hormone produced by male embryos, the paramesonephric ducts will develop into the fallopian tubes, uterus, upper one-third of the vagina and the cervix by default.[14] The normal length of the cervix in a premenopausal female who is not pregnant is 2 to 3 cm. Complications of a shorter cervix, otherwise known as cervical insufficiency, and its effect on the normal progression of pregnancy are discussed in another topic elsewhere.