Uterus

The uterus, also called the womb, is a reproductive organ found in most female mammals, including humans. Its main job is to house and support a fertilized egg as it grows into an embryo and then a fetus until birth. This organ responds to hormones and has glands in its inner lining that produce a nourishing fluid called uterine milk for the developing embryo. The term "uterus" is also used for similar structures in some animals that are not mammals.

In humans, the lower part of the uterus narrows into a section called the isthmus, which connects to the cervix—the gateway to the vagina. The upper part, known as the body of the uterus, connects to the fallopian tubes at the uterine horns. Above these openings is a rounded top called the fundus. The spot where the uterine cavity meets a fallopian tube is the uterotubal junction. A fertilized egg travels down a fallopian tube to the uterus, dividing along the way into a blastocyst. This blastocyst then implants into the uterine lining, called the endometrium, where it gets nutrients and develops into an embryo and later a fetus throughout pregnancy.

During human development in the womb, the uterus forms from the paramesonephric ducts, which fuse into a single organ called a simplex uterus. In many other animals, the uterus takes different shapes; for instance, some have two separate uteri, known as a duplex uterus.

In medical and professional settings, the term "uterus" is always used, while the word "womb" is more common in everyday speech. Events happening inside the uterus are described with the phrase "in utero."

**Structure**

In humans, the uterus sits in the pelvic region, just behind and slightly over the bladder, and in front of the sigmoid colon. It is pear-shaped, about 7.6 cm (3.0 inches) long, 4.5 cm (1.8 inches) wide, and 3.0 cm (1.2 inches) thick. A typical adult uterus weighs around 60 grams. Anatomically, it has four regions: the fundus (the rounded top above the fallopian tube openings), the body, the cervix, and the cervical canal. The cervix extends into the vagina. The uterus is held in place by ligaments made of endopelvic fascia, including the pubocervical, cardinal, and uterosacral ligaments. It is also covered by a sheet of peritoneum called the broad ligament.

**Layers**

The uterine wall has three layers, from inside out: the endometrium, myometrium, and perimetrium. The endometrium is the inner epithelial layer with a mucous membrane. It has a basal layer and a functional layer; the functional layer thickens and then sheds during the menstrual or estrous cycle. During pregnancy, the uterine glands and blood vessels in the endometrium grow larger and more numerous, forming the decidua. Vascular spaces fuse and connect to create the placenta, which supplies oxygen and nutrients to the embryo and fetus. The myometrium is mostly smooth muscle; its innermost part is the junctional zone, which thickens in a condition called adenomyosis. The perimetrium is a serous layer of visceral peritoneum covering the uterus's outer surface. Around the uterus is a layer of fibrous and fatty connective tissue called the parametrium, which links the uterus to other pelvic tissues. Commensal and mutualistic organisms live in the uterus, forming the uterine microbiome.

**Support**

The uterus is mainly supported by the pelvic diaphragm, perineal body, and urogenital diaphragm. Secondary support comes from ligaments, including the peritoneal ligament and the broad ligament.

**Major Ligaments**

Several peritoneal ligaments hold the uterus in place, the most important being pairs of the following: (list not fully detailed in source).

**Axis**

Normally, the human uterus is in anteversion and anteflexion. In most women, the long axis of the uterus tilts forward relative to the long axis of the vagina, against the bladder—this is anteversion. Also, the long axis of the uterus's body bends forward at the internal os relative to the cervix's long axis—this is anteflexion. About 50% of women have an anteverted uterus, 25% have a retroverted uterus, and 25% have a midposed uterus.

**Position**

The uterus sits in the middle of the pelvic cavity, in the frontal plane, due to the broad ligament. The fundus does not rise above the linea terminalis, and the vaginal part of the cervix does not go below the interspinal line. The uterus is mobile: it moves backward when the bladder is full, forward when the rectum is full, and upward if both are full. Increased pressure in the abdomen pushes it downward. This mobility comes from a musculofibrous apparatus with suspensory and sustentacular parts. Normally, the suspensory part keeps the uterus in anteflexion and anteversion (in 90% of women) and lets it "float" in the pelvis. The sustentacular part supports the pelvic organs, with the larger pelvic diaphragm in the back and the smaller urogenital diaphragm in the front. Pathological changes in the uterus's position include fixed retroversion or retroflexion, hyperanteflexion (tipped too far forward, often congenital or from tumors), anteposition, retroposition, lateroposition (the whole uterus shifts, caused by parametritis or tumors), and elevation, descensus, or prolapse.