Anatomy, Abdomen and Pelvis: Uterus

Structure and Function

The uterus supports implantation and fetal development. After fertilization, the ovum travels through the fallopian tube and implants into the endometrium, which supplies nourishment through specialized blood vessels. As the embryo grows, the uterus expands to accommodate the fetus. During labor, coordinated uterine contractions and cervical dilation facilitate delivery.

Embryology

Uterine development begins early and follows a complex sequence. Around the 8th week of gestation, primordia for both female and male internal genitalia—namely, the paramesonephric (Müllerian) and mesonephric (Wolffian) ducts—emerge. Sexual differentiation proceeds through the influence of growth factors, hormonal signaling, and genetic regulation.

The paired paramesonephric ducts extend caudally and reach the urogenital sinus, projecting into its posterior wall to form the Müllerian tubercle. Each duct has 3 segments: a vertical portion running parallel and lateral to the Wolffian duct, a horizontal segment crossing anterior to the ipsilateral Wolffian duct, and a 2nd vertical portion that fuses with its contralateral counterpart at the midline. The first 2 segments form the fallopian tube, while the fused 3rd segment undergoes canalization to form the uterus, cervix, and upper 1/5 of the vagina.

In female embryos, the absence of a Y chromosome and lack of testicular testosterone allow for uninterrupted development of the paramesonephric ducts, which fuse and canalize in the midline to give rise to the female pelvic organs. The mesonephric ducts regress simultaneously. Disruptions during this phase may result in a spectrum of Müllerian anomalies.

Blood Supply and Lymphatics

The uterus receives its primary blood supply from the uterine and ovarian arteries, both originating from the anterior branch of the internal iliac artery (see Image. Female Reproductive System Blood Supply). The uterine arteries serve as the main source of perfusion. Upon entering the myometrium, these vessels branch into arcuate arteries, which give rise to radial arteries. As they reach the endometrium, the radial arteries further divide into basal and spiral arteries.[3] These branches ensure adequate perfusion throughout the uterine wall and play a critical role in sustaining endometrial function during menstruation and supporting placental circulation during pregnancy.

Lymphatic drainage of the uterus follows a segmental pattern. The fundus primarily drains into para-aortic lymph nodes, mirroring the drainage pathways of the ovaries and fallopian tubes. Additional lymphatic channels follow the round ligament to reach the superficial inguinal lymph nodes. The lower uterine segments drain along the uterine vasculature toward external and internal iliac lymph nodes.

Nerves

The internal pelvic organs receive autonomic innervation from both sympathetic and parasympathetic divisions. Sympathetic input to the uterus arises from spinal levels T11 and T12 via the hypogastric plexus, while parasympathetic fibers originate from S2 to S4. The uterus and cervix are sensitive to distension, and cervical dilation during labor contributes significantly to labor pain.[4] Sensory signals from the uterus travel through the hypogastric nerves to spinal levels T10 to L1, resulting in referred pain to the lower back and abdomen. Sensory input from the vagina reaches the sacral spinal cord (S2 to S4) via pelvic nerves, producing referred pain in the gluteal region and posterior leg. The cervix transmits pain signals through both the hypogastric and pelvic nerves.[5][6]

Muscles

The uterus lies between the urinary bladder anteriorly and the rectum posteriorly. In women, the average uterine dimensions measure approximately 8 cm in length, 5 cm in width, and 4 cm in thickness, with a cavity volume ranging from 80 mL to 200 mL. Anatomically, the uterus divides into 3 main segments: the body, cervix, and fundus.

The wall of the uterus consists of 3 distinct layers. The endometrium forms the inner lining and includes a superficial functional layer that responds to reproductive hormones and a deeper basal layer. Shedding of the functional layer results in menstrual bleeding, while damage to the basal layer may lead to intrauterine adhesions and fibrosis, known as Asherman syndrome. The myometrium, composed of smooth muscle cells, forms the middle muscular layer and contributes to uterine contractions during labor. The outermost layer, the serosa or perimetrium, consists of a thin layer of epithelial cells.

Physiologic Variants

The long axis of the cervix typically does not align with the long axis of the uterine body. In most cases, the body of the uterus tilts forward over the cervix, a position known as anteflexion. A backward tilt at this junction is called retroflexion. In approximately 80% of women, the uterus lies at a right angle to the vagina with a forward tilt, a position known as anteversion. In 20% of women, the uterus tilts backward over the vagina, referred to as retroversion.[7] Uterine orientation within the pelvis varies among individuals. An anteverted uterus tilts anteriorly, while a retroverted uterus angles posteriorly. Descriptions of uterine position may also reference the angle of the fundus, with anteflexion indicating a forward tilt of the fundus and retroflexion indicating a posterior tilt.

The size and shape of the uterus vary throughout a woman's life, influenced by reproductive phase and hormonal changes. In prepubertal girls, the uterus is small, and the cervix is longer than the body, with a cervix-to-body ratio of 2:1. During the reproductive years, the uterus reaches its mature size, with the body larger than the cervix, resulting in a cervix-to-body ratio of 1:2. After menopause, the uterus atrophies, and the body becomes smaller than the cervix, restoring the cervix-to-body ratio to 2:1. Additionally, a nulliparous uterus tends to be smaller than a multiparous uterus. As women approach menopause, the lack of hormonal stimulation and cessation of menstruation contribute to uterine atrophy.[8][9]

Surgical Considerations

Uterine anomalies of clinical significance can markedly impact fertility and pregnancy. Early identification and appropriate surgical intervention are crucial to prevent complications such as miscarriage, preterm birth, and infertility.[10]

Segmental hypoplasia refers to underdevelopment or agenesis of parts of the uterus, such as the cervix, vagina, or fundus. This condition can result in a small uterus with a body-to-cervix ratio resembling that of a prepubertal girl. Segmental hypoplasia may lead to infertility or recurrent miscarriage due to inadequate uterine development. (Source: Gliozheni and Gliozheni, 2021)

A unicornuate uterus occurs when one of the Müllerian ducts fails to develop, resulting in a single uterine horn. A rudimentary horn is present in most cases, which can be noncommunicating or communicating with the main uterine cavity. Noncommunicating horns may cause symptoms like dysmenorrhea, hematometra, and endometriosis due to menstrual blood accumulation.

Uterine didelphys is a rare anomaly resulting from complete nonfusion of the Müllerian ducts, leading to 2 separate uterine cavities, cervices, and often a longitudinal vaginal septum. While many individuals remain asymptomatic, the condition is associated with a higher risk of miscarriage, preterm labor, and breech presentation during pregnancy.

A bicornuate uterus is characterized by a heart-shaped uterine cavity due to incomplete fusion of the Müllerian ducts. This anomaly can lead to infertility, recurrent miscarriage, and preterm labor. The severity varies, with some cases presenting a deep indentation at the uterine fundus.

A septate uterus is the most common Müllerian anomaly, resulting from incomplete resorption of the uterovaginal septum. The condition presents as a single uterine cavity divided by a fibrous or muscular septum. Septate uterus is associated with a significantly increased risk of miscarriage, preterm birth, and infertility. Surgical resection of the septum can improve reproductive outcomes.

Exposure to diethylstilbestrol (DES) during fetal development can lead to a T-shaped uterine cavity. This anomaly is associated with infertility, recurrent miscarriage, and preterm labor. The T-shaped uterus results from abnormal development of the endometrial lining, leading to a constricted uterine cavity.

Clinical Significance

The uterus may be involved in various gynecologic disorders, including prolapse, fibroids, polyps, infections, cancer, malformations, and adhesions. Hysterectomy is the 2nd most common surgical procedure for women in the U.S., following cesarean section. The most common uterine pathologies are discussed below.

A polyp is a well-defined, localized overgrowth of endometrial glands and stroma that may contain fibrous tissue or smooth muscle and projects into the endometrial cavity.[11] The lesion may be asymptomatic or present with painless abnormal uterine bleeding.[12] Small, asymptomatic polyps can resolve spontaneously, while progestins and gonadotropin-releasing hormone agonists may help alleviate symptoms. Surgical removal remains the definitive treatment for symptomatic cases.

Leiomyoma, or fibroid, is the most common benign tumor in women, often presenting as multiple discrete tumors within the uterine cavity. Leiomyoma is more prevalent in African-American women and is an estrogen-sensitive smooth muscle tumor. Although benign, the lesions have a slightly elevated risk of transforming into malignant leiomyosarcoma. These tumors tend to grow during pregnancy and shrink after menopause. Leiomyomas may cause abnormal uterine bleeding, miscarriages, and severe bleeding, leading to iron deficiency anemia in younger women.[13] The lesions often distort the shape of the uterus and can compress the rectum, resulting in constipation. Oral medications are typically ineffective, and surgical interventions, such as hysterectomy or myomectomy, are considered the gold standard for treatment.

Endometrial hyperplasia occurs due to excessive estrogen stimulation, leading to abnormal proliferation of endometrial glands. This condition is associated with an increased risk of endometrial carcinoma and requires careful monitoring and management.

Adenomyosis involves the extension of endometrial tissue, specifically glandular tissue, into the uterine myometrium, caused by hyperplasia of the basal layer of the endometrium. This condition commonly presents with dysmenorrhea and menorrhagia, and the uterus typically appears enlarged, uniform, and globular on imaging.[14] Medical treatment options include nonsteroidal anti-inflammatory drugs and hormonal therapy, such as combined estrogen-progestin pills, patches, and vaginal rings. Hysterectomy is considered the definitive treatment when other options fail to provide relief.

Endometritis is inflammation of the endometrium, typically associated with retained products of conception after delivery, miscarriage, or abortion, or with the presence of a foreign body. The retained material in the uterus can harbor bacterial flora from the vagina or intestinal tract, promoting infection.

Endometriosis occurs when nonneoplastic endometrial glands or stroma are found outside the endometrial cavity. The most common locations are the ovaries, pelvis, and peritoneum. An endometrioma can form in the ovaries, often described as a blood-filled "chocolate cyst." Evidence suggests retrograde menstruation as the likely cause of this condition.[15] On ultrasound, endometriosis appears as a homogeneous echotexture with hypoechoic areas. Medical treatments include nonsteroidal anti-inflammatory drugs, gonadotropin-releasing hormones, progestin therapy, and aromatase inhibitors. In cases where infertility is an issue, surgery to remove endometriosis implants while preserving the uterus and ovaries may improve the chances of conception.

Uterine cancer is the most common gynecologic cancer in developed countries and is strongly associated with excess estrogen.[16] Exogenous sources of estrogen, such as tamoxifen use or unopposed estrogen replacement therapy, contribute to an increased risk. Tamoxifen acts as an estrogen antagonist in breast tissue but also stimulates estrogen receptors in the endometrium, thereby raising the risk of endometrial cancer. Endogenous sources of estrogen include conditions such as polycystic ovary syndrome, obesity, and estrogen-secreting tumors, such as granulosa cell tumors. The majority of women with endometrial cancer present with abnormal vaginal bleeding, especially postmenopausal bleeding. Less frequently, patients may experience abdominal pain, changes in bowel habits, weight loss, and bloating.