Uterus: Anatomy, Function, and Conditions

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.

This article discusses the anatomy and function of the uterus and normal variations from person to person. It also explains some health conditions that can affect the uterus.

The uterus is usually the size of an apple but can stretch to the size of a watermelon during pregnancy. There are some conditions that may cause an enlarged uterus such as cancer, fibroids, and polycystic ovary syndrome.

Three distinct layers of tissue comprise the uterus:

There are four main parts of the uterus: 

The uterus can lie in different positions in different people. The positioning can be described based on the tilt of the uterus as well as the position of the fundus. The positions of the uterus include:

Fundus positioning may include:

The cervix dilates, or widens, to allow childbirth.

Shaped like an inverted pear, the uterus sits behind the bladder and in front of the rectum.

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

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

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.

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.

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.

By the 12th week of pregnancy, the uterus begins growing up into the abdomen, or stomach area, and fills the pelvic region. Throughout pregnancy, the uterus continues to grow up and out.

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.

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

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

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.

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. 

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:

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. 

A prolapsed uterus occurs when the uterus drops down into the vaginal area and sometimes pushes out of the vagina. This occurs when pelvic muscles and tissues are weak.

Risk factors for uterine prolapse include age, having given birth via vaginal delivery, going through menopause, being White, being overweight, and smoking.

Some individuals have a retroverted uterus 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.

There are two types of cancers that can affect the uterus:

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

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.