The Anatomy of the Uterus

Female genitalia, illustration
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The uterus, also known as the womb, is the hollow, pear-shaped organ in the female pelvis in which fertilization of an ovary (egg), implantation of the resulting embryo, and development of a baby take place. It is a muscular organ that both stretches exponentially to accommodate a growing fetus and contracts in order to push a baby out during childbirth. The lining of the uterus, the endometrium, is the source of the blood and tissue shed each month during menstruation.

Anatomy

Three distinct layers of tissue comprise the uterus:

  • Perimetrium: The outer layer of tissue made of epithelial cells
  • Myometrium: The middle layer made of smooth muscle tissue
  • Endometrium: The inner lining that builds up over the course of a month and is shed if pregnancy does not occur

Shaped like an inverted pear, the uterus sits behind the bladder and in front of the rectum. It has four main sections: 

  • Fundus: The broad curved area at the top and widest portion of the organ that connects to the fallopian tubes
  • Corpus: The main part of uterus that starts directly below the level of fallopian tubes and continues downward, becoming increasingly narrower
  • Isthmus: The lower narrow part of the uterus
  • Cervix: The lowest two inches of the uterus. Tubular in shape, the cervix opens into the vagina and dilates (widens) to allow

The uterus is supported in the pelvis by the diaphragm, the perineal body, and a collection of ligaments, including the round ligaments. 

Function

The uterus performs multiple important functions in the reproductive cycle, fertility, and childbearing. 

During a normal menstrual cycle, the endometrial lining of the uterus goes through a process called vascularization during which tiny blood vessels proliferate, leaving the lining thicker and rich with blood in the event the egg released during that cycle is fertilized. If this does not happen, the uterus sheds the lining as a menstrual period.

If conception occurs, the fertilized egg (the embryo) burrows into the endometrium from which the maternal portion of the placenta, the decidua basalis, will develop.

As a pregnancy progresses, the uterus grows and the muscular walls become thinner, like a balloon being blown up, to accommodate the developing fetus and the protective amniotic fluid produced first by the mother and later by urine and lung secretions of the baby.

During pregnancy, the muscular layer of the uterus begins contracting on-and-off in preparation for childbirth. These "practice" contractions, Braxton-Hicks contractions, resemble menstrual cramps; some women don't even notice them. They are not the increasingly powerful and regular contractions that are strong enough to squeeze the baby out of the uterus and into the vagina.

After a baby is born, the uterus continues to contract in order to expel the placenta. It will continue to contract in the coming weeks to return the uterus to its normal size and to stop the bleeding that occurs in the uterus during childbirth.

Associated Conditions

The uterus can be subject to any of a number of health issues. The most common uterine conditions include:

Endometriosis

An estimated 11% of women are affected by endometriosis, a condition in which the tissue of the endometrial lining grows outside of the uterus,causing symptoms that include painful cramps, chronic lower back pain, and pain during or after sex. Less common symptoms of endometriosis include spotting between periods, digestive problems, and infertility. 

Endometriosis typically is treated with extended-cycle hormonal birth control or an intrauterine device (IUD), though some women respond to complementary and alternative therapies such as acupuncture, chiropractic care, or supplements. The condition often goes away after menopause.

Fibroids

Uterine fibroids are non-cancerous tumors that grow in the muscular tissue of the uterus. Fibroids often do not cause symptoms or require treatment. 

For some women, however, uterine fibroids lead to heavy periods or pain, symptoms typically treated with over-the-counter pain relievers containing ibuprofen or acetaminophen or hormonal contraception. 

In severe cases, surgery such as endometrial ablation, myomectomy, or uterine fibroid embolization may be required. 

Uterine Polyps

Polyps are fingerlike growths that attach to the wall of the uterus. They can range in size from as small as a sesame seed to larger than a golf ball. Many women have polyps without knowing it. When symptoms do occur, they can include irregular periods, heavy bleeding, breakthrough bleeding, and infertility. 

Uterine polyps carry a small risk of cancer and should be removed with a procedure known as hysteroscopy. Sometimes a dilation and curettage (D and C) is done to remove and biopsy endometrial polyps. 

Tipped Uterus

Some women have a retroverted or retroflexed uterus, which means it is in a tipped or tilted position. This anatomical anomaly usually isn't detected unless a woman becomes pregnant, and usually is not an issue. .

However, some women with a tilted uterus may have a higher risk of miscarriage or experience a pregnancy complication known as uterine incarceration. If that happens, a Caesarean delivery will be necessary.

Uterine Cancer

There are two types of cancers that can affect the uterus: One, uterine sarcoma, is very rare. The other, endometrial cancer, originates in the endometrial lining and is fairly common. It typically occurs after menopause.

The primary symptom of endometrial cancer is abnormal vaginal bleeding, which may start as a watery, blood-streaked flow that gradually contains more blood. Abnormal vaginal bleeding is not a normal part of menopause and should be discussed with a gynecologist. 

Tests

Tests involving the uterus are used to screen for cancer, diagnose certain diseases and conditions, aid in fertility treatments, and monitor the progress of a pregancy. They include:

  • Pap smear: A test in which cervical cells are collected and analyzed in a lab to look for precancerous and other changes
  • Ultrasound: An imaging test that can be performed intravaginally (using a slender transducer—a wand-like instrument inserted into the vagina) or externally with a transducer applied to the abdomen. Ultrasound uses sound waves to produce images of the uterus, Fallopian tubes, ovaries, and surrounding tissue. In pregnancy, external ultrasound is used to check the baby’s progress.
  • Pelvic X-rays: An imaging test that uses radiation to take pictures of the pelvis. X-rays can be used to check the placement of the uterus and identify masses.
  • Hysteroscopy: An interventional procedure in which a tube is inserted into the cervix to see inside the uterus. Hysterectomy is often used to aid in the removal of fibroids.
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  11. U.S. National Library of Medicine. Hysteroscopy. Updated June 2, 2020.