Gross Anatomy and Microscopic Layers of the Uterus
The uterus is a hollow, thick-walled, muscular organ shaped like an inverted pear, nestled within the pelvic cavity between the bladder and the rectum. In a typical non-pregnant adult, it measures about 7.5 centimeters long, 5 centimeters wide, and 2.5 centimeters thick.
The uterine wall consists of three distinct microscopic layers:
- Perimetrium: The outermost serous layer, which is a continuation of the pelvic peritoneum covering the fundus and anterior/posterior body.
- Myometrium: The bulky middle layer made of interlacing smooth muscle fibers. It forms the functional muscle mass that contracts powerfully during labor and constricts blood vessels postpartum to prevent hemorrhage.
- Endometrium: The inner mucosal lining rich in glands and blood vessels. It has two zones: the functional layer, which sloughs off during menstruation, and the basal layer, which regenerates the functional layer each month.
Anatomical Landmarks: Fundus, Body, Isthmus, and Cervix
Clinicians divide the uterus into specific regional landmarks to accurately describe findings during pelvic examinations and monitor labor progress.
- Fundus: The dome-shaped upper portion of the uterus that lies above the openings of the fallopian tubes. Midwives use this landmark to measure fundal height during antenatal visits.
- Body (Corpus): The major central portion tapering downward from the fundus. It houses the uterine cavity and expands significantly during pregnancy to accommodate the growing fetus.
- Isthmus: A short, narrow constricted zone about 1 centimeter long situated between the body and the cervix. During late pregnancy, this region stretches and thins out to form the lower uterine segment.
- Cervix: The lower cylindrical neck of the uterus that projects into the vagina. It features an internal os opening into the uterine cavity and an external os opening into the vaginal vault.
Pelvic Support and the Ligamentous Network
The uterus is held in its anteverted and anteflexed position within the pelvic cavity by a coordinated system of active muscular floors and dense connective tissue ligaments.
- Broad Ligaments: Double folds of peritoneum stretching from the lateral sides of the uterus to the pelvic sidewalls, draping over the uterine tubes and ovaries.
- Round Ligaments: Fibromuscular cords running from the uterine horns through the inguinal canals to end in the labia majora, helping maintain the forward tilt of the uterus.
- Cardinal Ligaments (Mackenrodt's): Thick bands of connective tissue extending from the cervix and lateral vaginal vault to the lateral pelvic walls, representing the primary mechanical support preventing uterine prolapse.
- Uterosacral Ligaments: Fibrous bands passing backward from the cervix and upper vagina to the front of the sacrum, pulling the cervix upward and backward.
Arterial Supply and Venous Drainage Pathways
A rich, dynamic blood supply is vital for supporting the non-pregnant endometrial cycle and the immense vascular demands of the placenta during gestation.
The primary blood supply comes from the uterine artery, a major branch of the internal iliac artery. As the uterine artery approaches the side of the cervix, it crosses above the ureter—a critical anatomical relationship remembered by the surgical adage "water runs under the bridge." It then ascends along the lateral border of the uterus, sending off branching arcuate arteries that ring the organ, which further branch into radial and spiral arteries supplying the endometrium.
Venous blood mirrors this arterial path. The rich capillary networks drain into a dense plexus of uterine veins along the lateral margins, which ultimately empty into the internal iliac veins.
Lymphatic Drainage and Clinical Significance in Midwifery
The lymphatic drainage of the uterus follows distinct pathways corresponding to its anatomical regions, which plays a major role in the spread of pelvic infections or malignancies.
- Fundus and upper body: Drains mainly via lymphatic channels running inside the broad ligament directly into the lumbar (aortic) lymph nodes.
- Lower body and cervix: Drains into the internal iliac, external iliac, and sacral lymph nodes.
- Cervical area: Also drains laterally into the parametrial and obturator nodes.
In midwifery practice, understanding these pathways helps explain why pelvic infections like postpartum endometritis can track rapidly through tissue planes or why cervical pathologies present with specific regional lymph node enlargement.