The uterus is a pear-shaped dynamic organ that is responsible for a variety of functions such as gestation (pregnancy), menstruation, and labor and delivery. While most humans have one uterus, sometimes the development in utero may be incomplete, this is called a Mullerian Anomaly and can lead to many variants, ranging from a uterine septum to uterine didelphys (double uterus). In the normal anatomy, the uterus is located in the female pelvis immediately posterior to the bladder and anterior to the rectum. The female uterus is divided into three main anatomic segments (from superior to inferior): the fundus, corpus (body), and cervix (which protrudes into the vagina).
The uterus is supported by several ligaments including the utero ovarian ligament, round ligament, broad ligament, cardinal ligament, and uterosacral ligaments. It is further supported (inferiorly) by the pelvic diaphragm, urogenital diaphragm, and perineal body. The uterus may naturally lie in different positions such as anteverted/retroverted, anteflexed/retroflexed, or midline, and it may be rotated (especially during pregnancy). The uterus most commonly lies in an anteflexed and anteverted position in 50% of women.
When the uterus is in a retroverted/retroflexed or "tipped" position, it may cause pelvic pain, dyspareunia, minor incontinence, fertility difficulty, and difficulty inserting tampons. In pregnancy, this may lead to uterine incarceration.
The uterus is comprised of three tissue layers which include the following:
Adenomyosis is a benign condition that occurs when the endometrial layer invades into the myometrial layer, either focally or diffusely.
The uterus functions by accepting the fertilized ovum which passes through the fallopian tube. The ovum then implants into the endometrium where it receives nourishment from blood vessels developed exclusively for that purpose. As the embryo grows and matures, the uterus expands to accommodate the pregnancy. During normal labor, the uterus contracts as the cervix dilates, and this results in delivery of the infant.
The early development of the uterus is quite complex. At about eight weeks of gestation, primordia for both female and male internal genitalia [paramesonephric (Mullerian) and mesonephric (Wolffian)] ducts appear. The sexual differentiation process involves a series of steps which occur due to growth factors, hormonal signals, and inherited genetic influences.
In the female embryo, because of the absence of a Y chromosome and lack of testosterone from any testicular tissue, the normal sequence of developmental events result in canalization and fusion of the paramesonephric (Mullerian) ducts in the middle of the pelvis which gives rise to the female pelvic organs. At this time, the mesonephric (Wolffian) ducts regress. Any abnormality that occurs during this phase of development may result in a variety of paramesonephric anomalies.
The uterus receives blood from the uterine and ovarian arteries, which arise from the anterior branch of the internal iliac artery. As the blood supply enters the myometrium, it branches into the arcuate arteries, which branch into the radial arteries. As they enter the level of the endometrium, they branch into the basal and spiral arteries.
Nerves from T11 and T12 innervate the uterus. The sympathetic supply is from the hypogastric plexus, and the parasympathetic supply is from S2 to S4.
The uterus is located between the urinary bladder anteriorly and the rectum posteriorly. The average dimensions of the uterus in an adult female are 8 cm long, 5 cm across, and 4 cm thick. The uterine cavity has an average volume of 80 mL to 200 mL. The uterus is divided into three segments namely: the body, the cervix, and the fundus.
The anatomical position within the pelvis may vary. The normal position is an anteverted uterus, which is tipped forward, whereas a retroverted uterus is angled slightly posterior. The uterine position is also sometimes described in relation to the location of the fundus; that is, an anteflexed uterus, which is normal and where the fundus tilts forward. On the other hand, a retroflexed uterus is tilted posteriorly.
The uterus may also vary in size and shape depending on the reproductive phase of the female and response to the female sex hormones. For example, before puberty, the uterus is small, but during the reproductive years, the uterus may be quite large.
Also, a nulliparous uterus is usually smaller than a multiparous uterus. Furthermore, as the female reaches menopause, the uterus starts to atrophy chiefly due to lack of hormonal stimulation and loss of menstruation.
Uterine anomalies of clinical significance include:
The uterus is affected by a variety of gynecologic disorders including cancer, organ prolapse, fibroids, polyps, infections, malformations, and adhesions. Removal of the uterus is called a hysterectomy. This is the second most common surgical procedure performed on women in the United States ( a cesarean section is the most common).Uterine cancer is the most common gynecologic cancer in developed countries and is associated with excess estrogen. Exogenous sources of estrogen include tamoxifen use or unopposed estrogen replacement therapy. Tamoxifen increases the risk of endometrial cancer due to its ability to stimulate the estrogen receptors found on the endometrium. Endogenous sources of estrogen include polycystic ovary syndrome, obesity, and estrogen-secreting tumors such as granulosa cell tumors. The majority of women with endometrial cancer will present with abnormal vaginal bleeding or postmenopausal vaginal bleeding. Less commonly, they may present with abdominal pain, change in bowel habits, weight loss, and bloat.
Besides uterine abnormalities, the uterus may also be associated with several types of pathological disorders such as uterine fibroids, adenomyosis, endometrial hyperplasia, endometrial cancer, Asherman syndrome, uterine prolapse, and incontinence.