Anatomy of the cervix

Topography and parts of the uterus

The uterus (uterus, metra) is a unpaired hollow muscular organ. It is located in the lesser pelvis, occupying the space strictly between the urinary bladder and the rectum. Anatomically, the organ is divided into several parts:

• fundus — fundus uteri
• body — corpus uteri
• isthmus — isthmus uteri
• cervix — cervix uteri

Inside lies the uterine cavity (cavitas uteri), which has a characteristic triangular shape. Inferiorly, this cavity narrows and directly continues into the cervical canal (canalis cervicis uteri).

Structure of the cervix

The cervix has a total length of 3 to 4 cm. Its structure features two main zones:

1. The supravaginal part (portio supravaginalis).
2. The vaginal part (portio vaginalis).

Inside runs the cervical canal, the mucosa of which forms specific folds — plica palmatae. An important clinical landmark is the external os of the uterus (ostium uteri). Its shape varies depending on obstetric history: in nulliparous women, the os is round, whereas in multiparous women it transforms into a transverse slit.

Cervical dilation in labor

The mechanism of dilation depends on the patient's parity (number of previous births).

• In primiparae, the process proceeds strictly sequentially. First, the internal os dilates. Then the cervix thinned out (effacement occurs), and only after that does dilation of the external os begin.
• In multiparae, the external os opens almost simultaneously with the internal os. Concurrently with this process, shortening of the cervix itself takes place.

Complete dilation is defined as a state where the os is widened to 10–12 cm. The dynamics of changes in first-time labor are divided into stages:

• Stage A: the cervix is preserved; the isthmus and internal os are clearly differentiated.
• Stage B: onset of cervical effacement.
• Stage C: the cervix is fully effaced.
• Stage D: complete cervical dilation.

Fetal descent and amniotic fluid

In the first stage of labor, cervical dilation occurs simultaneously with the descent of the fetal presenting part through the birth canal. Descent of the fetal head into the pelvic cavity starts with the onset of contractions. By the time of complete dilation, the head is usually engaged by its large segment at the pelvic inlet or has already descended into the pelvic cavity.

As it descends, the head presses the wall of the lower uterine segment against the bony framework of the birth canal. This forms the internal girdle of contact — the zone where the uterine walls tightly embrace the fetal head. This girdle divides the amniotic fluid into two portions:

• Forewaters (located below the girdle).
• Hindwaters (located above the girdle).

By the time of complete dilation, the amniotic sac loses its physiological function and must rupture. Timely rupture of membranes is considered when dilation has reached 10 cm or is nearly complete (8 cm or more).

Internal sutures for lacerations

If soft tissue lacerations of the birth canal occur during labor, internal sutures are placed immediately after the birth of the fetus. Their localization includes the vaginal walls, the uterus itself, and the cervix. Anesthesia is mandatory when repairing the vaginal walls. Exclusively absorbable suture material is used, so subsequent removal of the stitches is not required.

Class and exam questions

What else to study on this topic

• Mechanism of fetal head descent into the pelvic cavity during contractions
• Division of amniotic fluid into fore- and hindwater portions
• Physiological function of the amniotic sac and timing of its rupture
• Technique of placing internal sutures for soft tissue lacerations of the birth canal
• Anesthesia during repair of the vaginal walls and cervix

Other topics in the section "Splanchnology / Urogenital System"

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.