The Anatomy of the Uterus

The uterus, also known as the womb, is a hollow, muscular organ located in the pelvis between the bladder and rectum of individuals who are assigned female at birth. This pear-shaped organ plays a role in menstruation, pregnancy, and childbirth.

The lining of the uterus (endometrium) is the source of the blood and tissue shed each month during menstruation. After an egg released from an ovary has been fertilized, the resulting embryo implants in the uterus and fetal development begins.

The uterus stretches exponentially to accommodate a growing fetus and contracts to push a baby out during childbirth.

Uterus 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 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. There are four main parts of the uterus: 

• 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 the 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 that is tubular in shape and opens into the vagina

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

Uterus Function

There are four main functions of the uterus. The uterus plays a significant role in the processes of:

• Menstruation
• Implantation of the embryo
• Gestation, or development of the embryo and later the fetus
• Labor

Menstruation

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

Implantation

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

Gestation

As a pregnancy progresses, the uterus grows and the muscular walls become thinner, like a balloon being blown up.

This helps accommodate the developing fetus and the protective amniotic fluid. Amniotic fluid is produced first by the pregnant individual and later by urine and lung secretions of the baby.

Labor

During pregnancy, the muscular layer of the uterus begins contracting on-and-off in preparation for childbirth. These "practice" contractions, or Braxton-Hicks contractions, resemble menstrual cramps; some individuals don't even notice them.

Unlike Braxton-Hicks contractions, labor contractions are increasingly powerful and 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.

Uterus Problems and Conditions

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

Endometriosis

An estimated 11% of assigned females are affected by endometriosis, a condition in which the tissue of the endometrial lining grows outside of the uterus. This can cause symptoms that include:

• Painful cramps
• Chronic lower back pain
• Pain during or after sex

Less common symptoms of endometriosis include spotting between periods, digestive problems, and infertility.

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

Fibroids

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

For some individuals, however, uterine fibroids lead to heavy periods or pain. These symptoms are 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 individuals have polyps without knowing it. When symptoms do occur, they can include:

• Irregular periods
• Heavy bleeding
• Breakthrough bleeding
• 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 Position

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

However, some individuals 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:

• Uterine sarcoma, which is very rare
• Endometrial cancer, which originates in the endometrial lining, is fairly common, and 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 healthcare provider. 

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: This is a test in which cervical cells are collected and analyzed in a lab to look for precancerous cells and other changes.
• Ultrasound: This imaging test 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 to check on the pregnancy. Ultrasound uses sound waves to produce images of the uterus, fallopian tubes, ovaries, and surrounding tissue.
• Pelvic X-rays: This imaging test uses radiation to take pictures of the pelvis. X-rays can be used to check the position of the uterus and identify masses.
• Hysteroscopy: This is a procedure in which a tube is inserted into the cervix to see inside the uterus. An operative hysteroscopy is often used to aid in the removal of fibroids.

Summary

The uterus is a hollow, pear-shaped organ located behind the bladder and in front of the rectum. The uterus is made up of three main tissues and has four distinct parts.

The uterus plays a significant role in menstruation, implantation, gestation, and labor. While the uterus is subject to certain health concerns, there are diagnostic tests and treatment options available.