The uterus is a muscular organ in the pelvis that plays a central role in menstruation, pregnancy, and childbirth. It is also part of the reproductive tract and connects with the fallopian tubes above and the vagina below through the cervix.
Although the uterus is often described simply as the organ where a pregnancy develops, that description leaves out much of its normal biology. Its tissues change throughout the menstrual cycle, its muscular wall is essential during labor, and several common conditions can alter its size, shape, or function.
Where the uterus is and what it looks like
The uterus normally sits in the pelvis between the bladder and the rectum. In an adult who is not pregnant, it is a relatively small, hollow organ with a shape often compared with an upside-down pear. Its exact position and orientation vary from person to person.
The uterus has two main regions: the body, or main upper portion, and the cervix, the narrower lower portion. The cervix extends into the upper vagina and contains a passage called the cervical canal. The uterus connects with the fallopian tubes near the upper corners of its body.
The uterine wall has three principal layers:
- Endometrium: The inner lining. It responds to reproductive hormones and undergoes predictable changes during the menstrual cycle. If pregnancy does not occur, the functional portion of this lining is shed during menstruation.
- Myometrium: The thick middle layer made primarily of smooth muscle. It expands substantially during pregnancy and contracts during labor.
- Perimetrium: The outer covering of the uterus, formed largely by a layer of tissue continuous with the abdominal lining in this region.
The uterus is supported within the pelvis by connective tissue, ligaments, and the surrounding pelvic structures. Its position is not fixed in exactly the same orientation in every person.
How the uterus functions during the menstrual cycle
The uterus changes in response to hormones produced by the ovaries and regulated through the brain and pituitary gland.
During the first part of the menstrual cycle, rising estrogen levels stimulate the endometrium to grow and thicken after the previous menstrual period. This creates a lining capable of supporting an embryo if pregnancy occurs.
After ovulation, progesterone produced by the ovary helps transform the endometrium into a more glandular, nutrient-supportive state. If an embryo does not implant, hormone levels fall. The functional layer of the endometrium then breaks down and is expelled as menstrual blood and tissue.
Menstrual bleeding therefore comes primarily from the uterus, not the ovaries. The amount and duration of bleeding vary considerably among individuals, and changes from a person’s usual pattern can sometimes signal an underlying condition.
What happens to the uterus during pregnancy
If fertilization occurs and the resulting embryo reaches the uterus, implantation normally takes place in the endometrium. The developing placenta forms an interface between maternal and fetal tissues and becomes an important source of hormonal and metabolic support for the pregnancy.
The uterus then undergoes major structural changes. Its muscular wall grows and stretches as the pregnancy progresses, while its blood supply increases substantially. The organ expands from a relatively small pelvic structure into an abdominal organ capable of accommodating the developing fetus, placenta, and amniotic fluid.
Near the end of pregnancy, coordinated contractions of the myometrium help open the cervix and move the fetus through the birth canal. After delivery, continued uterine contraction helps compress blood vessels at the site where the placenta was attached and assists the uterus in returning toward its nonpregnant size.
The uterus and the cervix are not the same thing
The cervix is the lower portion of the uterus, but it has distinct anatomy and functions.
Its canal connects the uterine cavity with the vagina. Cervical mucus changes under hormonal influence: around ovulation it generally becomes thinner and more conducive to the movement of sperm, while at other times it tends to be thicker.
The cervix also changes during pregnancy and labor. During childbirth, hormonal and mechanical changes cause it to soften, shorten, and gradually open, a process known as dilation. The cervix is also the site from which cervical cells are collected during cervical cancer screening.
Common conditions that affect the uterus
Many uterine conditions are benign, meaning they are not cancerous, but they can still cause significant symptoms. Others require more urgent evaluation.
Uterine fibroids
Fibroids, also called leiomyomas, are noncancerous growths arising from the muscular tissue of the uterus. They can occur within the uterine wall, project into the uterine cavity, or grow outward from the uterus.
Some fibroids cause no symptoms and are discovered incidentally. Others can cause heavy or prolonged menstrual bleeding, pelvic pressure, pelvic pain, frequent urination, or reproductive problems. Their effects depend partly on their size and location.
Treatment is not always necessary. When symptoms are significant, options can include medications, procedures that target the fibroids, or surgery. The appropriate approach depends on factors such as symptoms, the location and size of the fibroids, age, and whether future pregnancy is desired.
Endometriosis
Endometriosis occurs when tissue resembling the endometrium grows outside the uterus. It most commonly involves structures within the pelvis, although it can occur elsewhere.
This tissue responds to hormonal changes during the menstrual cycle and can produce inflammation and pain. Symptoms may include painful periods, pelvic pain, pain during sex, pain associated with bowel movements or urination during menstruation, and difficulty becoming pregnant.
Endometriosis is different from adenomyosis, although the conditions can overlap and cause similar symptoms.
Adenomyosis
In adenomyosis, tissue resembling the endometrium grows into the muscular wall of the uterus. This can make the uterus enlarged or tender and may cause heavy menstrual bleeding and painful periods.
The distinction from endometriosis is primarily anatomical: adenomyosis involves the uterine muscle, whereas endometriosis involves endometrium-like tissue outside the uterus.
Endometrial polyps
Endometrial polyps are growths that arise from the inner uterine lining. They are usually benign, although some can contain precancerous or cancerous changes.
Polyps may cause irregular bleeding, bleeding between periods, or bleeding after menopause. Some cause no symptoms at all. Evaluation may involve imaging or direct examination of the uterine cavity, depending on the circumstances.
Endometrial hyperplasia
Endometrial hyperplasia means that the uterine lining has become excessively thick because of an abnormal pattern of cell growth. It often develops when estrogen stimulates the endometrium without sufficient progesterone opposition.
Some forms of hyperplasia can progress to endometrial cancer, particularly when abnormal cellular changes are present. Abnormal uterine bleeding, especially unexpected bleeding after menopause, therefore deserves medical evaluation rather than being assumed to be a normal hormonal change.
Endometrial cancer
Endometrial cancer begins in the lining of the uterus and is the most common type of uterine cancer. Abnormal vaginal bleeding is an important warning sign, particularly bleeding after menopause.
Diagnosis generally requires examining endometrial tissue, often obtained through an endometrial biopsy. Treatment depends on the cancer’s type and extent and may involve surgery, radiation, systemic medication, or combinations of these treatments.
Endometrial cancer is distinct from cervical cancer, which begins in the cervix rather than the uterine body.
Abnormal uterine bleeding
Abnormal uterine bleeding is a symptom rather than a single diagnosis. It can include bleeding that is unusually heavy, prolonged, frequent, irregular, or occurs between expected periods. Bleeding after menopause is also considered abnormal.
Potential causes range from hormonal changes and contraceptive medications to fibroids, polyps, adenomyosis, pregnancy-related conditions, thyroid disorders, bleeding disorders, and cancers.
Evaluation usually begins with a medical history and pelvic examination when appropriate. Depending on age, symptoms, pregnancy status, and risk factors, clinicians may use pregnancy testing, blood tests, ultrasound, or sampling of the endometrium to identify the cause.
The pattern matters. A sudden or substantial change in bleeding, particularly when accompanied by dizziness, weakness, severe pain, or other concerning symptoms, warrants prompt medical attention.
How doctors examine the uterus
Several methods can be used to evaluate the uterus, and each answers different questions.
Pelvic examination can provide information about the cervix, vagina, and general position or size of the uterus, although it cannot visualize the uterine cavity directly.
Ultrasound is commonly used to examine the uterus and ovaries. Transvaginal ultrasound, in which a specialized probe is placed in the vagina, can provide detailed images of the uterus and endometrium.
Hysteroscopy uses a thin camera inserted through the cervix to view the inside of the uterine cavity directly. It can also allow certain abnormalities, such as some polyps or fibroids, to be treated during the same procedure.
Endometrial biopsy removes a small sample of the uterine lining for laboratory examination. It is particularly useful when clinicians need to investigate abnormal bleeding or assess the endometrium for abnormal cell growth.
Other imaging techniques, including magnetic resonance imaging, may be useful when a more detailed assessment of the uterus is needed.
When uterine symptoms deserve medical attention
Not every change in menstrual bleeding indicates disease, but persistent or unusual symptoms should not automatically be attributed to stress, aging, or “normal” hormonal fluctuations.
Medical evaluation is especially important for unexplained bleeding after menopause, unusually heavy or prolonged menstrual bleeding, recurrent bleeding between periods, persistent pelvic pain, or symptoms that interfere substantially with daily life. Pregnancy-related pain or bleeding can require urgent assessment because some pregnancy complications can be serious.
The uterus is a dynamic organ whose structure and function change throughout the reproductive years. Understanding its anatomy makes it easier to distinguish normal reproductive processes—such as cyclical changes in the endometrium—from symptoms that may warrant further evaluation.
