The uterus, or womb, is an essential organ in the female reproductive system. It is vital in menstruation, pregnancy, and overall reproductive health. This pear-shaped, muscular organ is located in the pelvis between the bladder and rectum1,2.
Understanding uterine anatomy and function is important for recognizing potential health concerns. The uterus can be affected by various conditions, like fibroids, endometriosis, and certain cancers. Awareness of normal uterine function helps women and healthcare professionals identify abnormal symptoms and seek timely medical attention.
In this article, we will cover the anatomy of the uterus, how it works, and when you should seek medical attention if you think there is a problem.
The Anatomy of the Uterus
Structure and Location
As mentioned above, the uterus is a pear-shaped hollow muscular organ located in the female pelvis. It measures approximately 7.5 - 8 cm in length, 5 cm in width at its upper part, and around 4 cm in thickness2. The average weight of an adult uterus is around 30 - 40 grams3.
The uterus is located in the pelvic cavity between the bladder and the rectum. It is held in place by several ligaments, including the broad, round, cardinal, and uterosacral ligaments4. These ligaments are part of the connective tissue network that lines the pelvis's walls and floor (endopelvic fascia)5.
The uterus can be divided into four main regions6:
- Fundus: The uppermost rounded portion above the openings of the fallopian tubes.
- Body: The main part of the uterus located below the fundus.
- Isthmus: A slight constriction between the body and the cervix.
- Cervix: The lower, narrow part that protrudes into the vagina.
Layers of the Uterus
The uterine wall has three layers7:
- The endometrium: The innermost later, which thickens and sheds during the menstrual cycle.
- The myometrium: The middle layer, composed primarily of smooth muscle.
- The perimetrium: The outer serous layer (smooth tissue membrane) of the visceral peritoneum.
Blood Supply and Nerve Connections
The uterus receives its primary blood supply from the uterine arteries, which support its functions. The uterine arteries are paired vessels typically originating from the anterior division of the internal iliac artery (blood vessels that provide blood to the legs, pelvis, and pelvic organs)8.
Contractions of the uterus are primarily controlled by electrical activity within myometrial cells, which are influenced by hormonal and neural factors.
- Electrical activity: Electrical signals in uterine muscle cells directly trigger contractions9.
- Hormonal influence: Estrogen and progesterone affect uterine muscle cell membrane potentials, modulating electrical activity10.
- Neural control: Parasympathetic nervous system activation, particularly through oxytocin-mediated pathways, stimulates contractions11.
- Oxytocin: Stimulates powerful uterine contractions during childbirth12.
- Stretch receptors: Uterine growth alters electrical signal propagation, affecting contractility13.
The interplay between these electrical, hormonal, and neural factors allows for the precise control of uterine contractions, which are necessary for various reproductive functions, including menstruation, implantation, and childbirth.
The Function of the Uterus
Menstrual Cycle and Hormonal Influence
The uterus undergoes a series of changes each month to prepare for potential pregnancy. These changes are driven by hormonal fluctuations, primarily estrogen and progesterone. This process is part of the menstrual cycle, which typically lasts about 28 days but can vary from 18 to 43 days14,15.
There are four phases to the menstrual cycle16.
Menstruation: The cycle begins with the shedding of the uterine lining, lasting 3-7 days.
Follicular Phase: This phase starts on the first day of menstruation and lasts 13-14 days. During this time, estrogen levels rise, thickening the uterine lining.
Ovulation: Around day 14, an egg is released from the ovary.
Luteal Phase: This phase prepares the uterus for potential pregnancy.
A number of hormones are involved in uterine changes during the menstrual cycle.
Estrogen levels increase during the follicular phase, stimulating the growth and thickening of the endometrium17. It also promotes the growth of endometrial cells, increasing uterine thickness.
After ovulation, progesterone levels rise, produced by the corpus luteum, further preparing the uterine lining for potential implantation by thickening the lining and enriching it with blood vessels to provide nutrients for a potential embryo18.
If pregnancy doesn’t occur, progesterone levels drop, leading to the breakdown and shedding of the uterine lining, marking the beginning of a new cycle19. This hormonal interplay ensures that the uterus is optimally prepared for pregnancy each month, demonstrating the remarkable adaptability of the female reproductive system.
Pregnancy and Fetal Development
If pregnancy does occur, then the uterus undergoes several changes to accommodate the growing fetus.
- By 12 weeks, the uterus is the size of a grapefruit and fills the pelvis20.
- At 20 weeks, the uterus can be felt at the level of the belly button, with the uterine wall thinning to 3-5 mm (down from 4 cm).
- By the end of the pregnancy, the uterus can stretch to the size of a watermelon.
The uterus expands during pregnancy, growing from 1 ounce to about 2 pounds at birth21.
The placenta, forming 7-10 days after conception, is important for fetal development22. It acts as an interface between maternal and fetal bloodstreams, allowing nutrients and oxygen to pass from mother to baby without direct blood mixing23. It also filters out harmful waste, produces essential hormones like lactogen, estrogen, and progesterone, and passes antibodies to the baby for early immunity24.
Postpartum and Uterine Recovery
After childbirth, the uterus undergoes a process called involution to return to its pre-pregnancy size. This involves contractions, often felt as afterpains, which are strongest in the first few days postpartum25. Oxytocin stimulates these contracts and aids in uterine shrinkage. The uterus rapidly decreases in size, going from filling the entire pelvic cavity to about the size of a pear within six weeks26.
Hormonal changes significantly influence postpartum recovery. Estrogen levels drop sharply after delivery, remaining low for several weeks, especially during breastfeeding27. Prolactin increases to support milk production, while progesterone levels initially decrease before gradually rising again28,29.
Common Uterine Problems
Several common conditions can affect the uterus, potentially impacting reproductive health.
Fibroids are benign tumors made of smooth muscle that develop in the uterine wall. They affect up to 60 - 80 percent of women by age 50 and can cause heavy menstrual bleeding, pelvic pain, and pressure30.
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus31. It can cause severe menstrual cramps, pain during sex, and infertility.
Adenomyosis involves the growth of endometrial tissue into the muscular wall of the uterus31. It affects women in their later reproductive years and can lead to bleeding and painful periods.
Uterine polyps are abnormal overgrowths of the endometrial lining that extend into the uterine cavity32. They are more common in pre-and postmenopausal women and can cause abnormal bleeding.
What Are The Symptoms of an Abnormal Uterus?
Symptoms can include33–35:
- Heavy menstrual bleeding (soaking through protection every 1-2 hours)
- Irregular periods (less than 28 days apart or more than 35 days apart)
- Prolonged bleeding (more than 7 days)
- Spotting between periods
- Severe pelvic pain and cramping
- Pain during sex.
- Fertility issues or difficulty conceiving
Other symptoms might include:
- Fatigue due to anemia from blood loss.
- Abdominal pressure or fullness.
- Lower back pain.
- Urinary or bowel problems, caused by pressure on nearby organs.
The severity of these symptoms can vary, and many of them overlap with other issues.
Ensuring Uterine Health
How Do I Check if My Uterus is Healthy?
Regular gynecological exams are important for maintaining reproductive health. Annual check-ups can detect conditions like cervical cancer or STIs before symptoms arise. They also allow your healthcare provider to monitor changes in your reproductive system, provide health education, and address concerns about birth control, fertility, or menopause. Even without noticeable symptoms, yearly well-woman exams are important for early detection and preventive care.
To gauge your uterine health, monitor your menstrual cycles for regularity and normal flow. Watch for severe pelvic pain, unusual discharge, or pain during intercourse. Healthy habits like diet, exercise, and stress management are key.
A balanced diet rich in fruits, vegetables, and omega-3s can help36. Regular exercise promotes hormonal balance, while relaxation techniques like yoga or meditation can reduce tension. Prioritize sleep and self-care, and maintain a healthy weight. Remember, regular gynecological check-ups are vital for early detection and continued well-being37.
How Imaging Helps Diagnose Uterine Conditions
Imaging plays an important role in diagnosing uterine conditions:
Ultrasound: This first-line imaging test provides detailed views of the uterus and identifies fibroids, polyps, or other abnormalities38.
Magnetic Resonance Imaging (MRI): Offers superior soft tissue contrast, allowing for precise mapping of fibroids and differentiation between various uterine conditions39.
These imaging techniques help determine the size, location, and number of uterine abnormalities, guiding treatment decisions.
Summary: The Uterus: Anatomy and Function
The uterus is crucial in menstruation, pregnancy, and overall reproductive health. Understanding its anatomy and how it works can help you identify when something doesn’t feel right. A number of common conditions can affect uterine health, making regular monitoring and imaging tests key to early detection and treatment.
If you want to be proactive about your uterine health, why not book an Ezra full-body MRI? Our annual scan catches potential cancer earlier, leveraging AI through the screening process to make it more efficient, affordable, and faster.
