What is the uterus?
The uterus is a muscular organ in the shape of an upside-down pear. Inside the uterus is the uterine cavity (cavum uteri) with a flat, triangular interior. The upper two-thirds of the uterus is called the body of the uterus (corpus uteri) with the dome (fundus uteri) in the uppermost region, which overhangs the outlet of one fallopian tube each on the right and left. The lower, narrowly tapering third is called the cervix uteri.
Between the corpus uteri and the cervix is a narrow connecting piece (isthmus uteri), which is about half a centimeter to a whole centimeter long. Although this area anatomically belongs to the cervix, its interior is lined with the same mucosa as the corpus uteri. However, the mucosa in the isthmus – unlike that in the body of the uterus – does not participate in the cyclical changes within the menstrual cycle.
The uterus is normally slightly bent forward (anteversion) and slightly bent forward in relation to the cervix (anteflexion). It rests on the urinary bladder in this way. Depending on the filling of the urinary bladder, the uterus shifts a little.
Uterus size and weight
The uterus size is about seven to ten centimeters in an adult, non-pregnant woman. The uterus is one and a half to three centimeters thick and weighs about 50 to 60 grams. This weight can increase to about one kilogram during pregnancy.
Structure of the uterine wall
The wall structure in the uterus shows three layers: The outer layer is a lining with peritoneum, the connective tissue perimetrium. Towards the inside follows a thick layer of muscle cells called myometrium. On the very inside lies a mucous membrane. In the uterine cavity, this is called the endometrium. It differs in structure from the mucous membrane in the cervix.
The uterine function only comes into play during pregnancy: the uterus provides the space in which the fertilized egg develops into a viable child.
The uterus prepares for this task every month: The endometrium thickens to a thickness of about six millimeters in the first half of the cycle under the influence of hormones (estrogens). In a further step, the hormone progesterone unfolds its effect: it prepares the endometrium for the implantation of a potentially fertilized egg. If fertilization has not taken place, the thickened mucous membrane is shed and excreted via menstrual bleeding (the blood from ruptured mucosal vessels). During this process, the strong muscular layer inside the uterus contracts to expel the rejected tissue to the outside. These muscle contractions can be perceived as period pains of varying intensity.
Where is the uterus located?
The uterus is located in the woman’s lesser pelvis between the urinary bladder and the rectum. The perimetrium extends from the upper end to the anterior surface of the uterus, which rests on the urinary bladder, and further down to the isthmus, where it continues onto the urinary bladder. In the posterior part of the uterus, the perimetrium rests on the uterus down to the cervix.
The uterus is held in position by various connective tissue structures (retaining ligaments). In addition, the pelvic floor muscles normally prevent the uterus from descending.
What problems can the uterus cause?
In endometriosis, the lining of the uterus (endometrium) also grows outside the uterus, for example in the fallopian tubes, ovaries, vagina, peritoneum or – although rarely – in regions outside the genital area, for example in the groin, rectum, lymph nodes, lungs or even brain. These endometrial foci also participate in the menstrual cycle, so they are built up and broken down cyclically (including a small amount of bleeding that is absorbed by the surrounding tissue). Common symptoms of endometriosis include abdominal pain, cyclical back pain, pain during sex, menstrual irregularities and infertility.
The uterus may descend (i.e., enter deeper into the pelvis), usually along with the vagina. Due to the tight connective tissue connections, the neighboring organs of the urinary bladder and/or rectum are also carried along. This descent (descensus) of the pelvic organs is a progressive process. Eventually, the uterus may partially or completely exit the vagina (prolapse). Risk factors for pelvic organ descensus include weakness or injury to the pelvic floor (such as birth injuries), obesity, chronic cough, and chronic constipation.
A cancerous growth in the cervix is called cervical cancer (cervical carcinoma). Risk factors include early first sexual intercourse, frequently changing sexual partners, and poor genital hygiene. These factors increase the risk of infection with human papilloma viruses (HPV). These germs are involved in the development of cervical carcinoma.
Uterine polyps result from estrogen-induced hyperplasia (enlargement/increased growth) of the endometrial tissue. Uterine fibroids are benign muscular growths in or on the uterus whose growth is determined by estrogen. Both polyps and fibroids can cause discomfort, but do not have to.
