Uterus Anatomy

Key facts

Location and relations

The uterus lies in the middle of the pelvis with the bladder in front and the rectum behind. Between uterus and rectum the peritoneum dips into the recto-uterine pouch of Douglas, the lowest point of the peritoneal cavity when standing, where blood from a ruptured ectopic pregnancy or pus from pelvic infection collects. Usually the organ is tilted forward over the bladder (anteverted) and bent forward at the cervix (anteflexed); a backward-tilted uterus is a normal variant. It is held up from below by the levator ani muscles of the pelvic floor and at the sides and back by the cardinal and uterosacral ligaments attached to the cervix. The broad ligament is a peritoneal fold carrying the vessels, and the round ligament runs through the inguinal canal to the labium majus, keeping the uterus anteverted. The ureter passes close to the cervix.

Parts and layers

Above the openings of the fallopian tubes lies the rounded fundus; below it the body narrows through the isthmus into the cervix, a firm cylinder about 2.5 cm long that projects into the top of the vagina. The endocervical canal runs from the internal os to the external os and is lined by mucus-secreting columnar epithelium, while the outer surface of the cervix is covered by squamous epithelium. Where the two meet, the transformation zone, is where most cervical cancers begin and where the cells for cervical screening are taken. The wall has three layers: the endometrium, whose functional layer thickens each cycle and is shed at menstruation while the basal layer regenerates it; the myometrium, a thick coat of interlacing smooth muscle that forms most of the organ; and the perimetrium, the visceral peritoneum covering the body and fundus.

Blood supply and lymph

Each uterine artery arises from the internal iliac artery, runs medially in the base of the broad ligament and crosses above the ureter close to the cervix, remembered as 'water under the bridge' and the reason the ureter is at risk during hysterectomy. It then climbs tortuously up the side of the uterus, sending branches into the myometrium that end as spiral arterioles in the endometrium; constriction of these vessels starts menstruation. Near the tube it joins the ovarian artery, a direct branch of the abdominal aorta, so the uterus has two sources of supply. Veins form a plexus in the broad ligament draining to the internal iliac veins. Lymph from the cervix and body reaches the internal and external iliac nodes, lymph from the fundus follows the ovarian vessels to the para-aortic nodes, and a few vessels run with the round ligament to the groin.

Innervation

The uterus is supplied by the uterovaginal plexus, an extension of the inferior hypogastric plexus lying on either side of the cervix. Sympathetic fibres reach it from the lower thoracic and upper lumbar segments through the hypogastric nerves, and parasympathetic fibres from S2 to S4 through the pelvic splanchnic nerves. Pain from the body of the uterus travels with the sympathetic fibres and is felt in the lower abdomen and lower back, whereas pain from the cervix travels with the parasympathetic fibres and is felt over the sacrum and perineum, which is why period pain and labour pain spread to the back. Contractions in labour are driven mainly by hormones rather than nerves, so labour still proceeds after spinal cord injury, and an epidural blocks the pain pathways without stopping the uterus from working.

Function and hormonal control

Under oestrogen from the growing follicle the endometrium proliferates, and after ovulation progesterone from the corpus luteum makes it secretory and receptive. If no embryo implants, the corpus luteum fails, hormone levels fall and the functional layer is shed as a period lasting a few days to a week. If fertilisation occurs in the tube, the embryo implants in the endometrium about a week later; its trophoblast makes hCG, the hormone detected by a pregnancy test, which keeps the corpus luteum working until the placenta takes over. Through pregnancy the myometrium grows by hypertrophy and the uterus rises out of the pelvis, reaching the umbilicus at around 20 weeks and the xiphisternum by about 36 weeks. In labour, oxytocin drives contractions in a positive feedback loop while the cervix softens, thins (effaces) and dilates; afterwards the uterus shrinks back over about six weeks.

Changes across life

Before puberty the uterus is small and the cervix makes up much of its length; under rising oestrogen at puberty the body grows and the first period (menarche) follows. Pregnancy stretches the myometrium and the cervix, and each birth leaves the organ slightly larger. After the menopause the uterus shrinks, the endometrium becomes thin and periods stop, so any bleeding after the menopause needs prompt assessment. In trans men and non-binary people taking testosterone, periods usually stop within a few months, but bleeding can continue or return, and any unexpected bleeding needs assessment. The uterus stays in place unless a hysterectomy is chosen as part of gender-affirming surgery, sometimes with removal of the ovaries, which brings on a surgical menopause and raises the same bone health questions. Hysterectomy may also be done before phalloplasty, where a vaginectomy is usually needed if the urethra is lengthened. A retained cervix still needs cervical screening, and testosterone is not a contraceptive.

Clinical relevance

Fibroids, benign smooth-muscle growths of the myometrium, are the commonest uterine tumour and a major cause of heavy periods, which can lead to iron deficiency anaemia. Endometriosis is endometrium-like tissue growing outside the uterus, and adenomyosis is its growth into the myometrium; both cause pelvic pain. Persistent infection with high-risk HPV can turn cells of the transformation zone into cervical cancer. Cervical screening, offered in the UK by invitation between the ages of 25 and 64 to everyone with a cervix, including trans men and non-binary people, is not a test for cancer: it looks for high-risk HPV and for the cell changes that come before cancer, so they can be treated. People registered as male with their GP practice may need to ask for it, and symptoms such as bleeding after sex or after the menopause need examination rather than screening. Weakened supports allow the uterus to prolapse into the vagina. A caesarean section opens the lower segment of the uterus to deliver the baby; hysterectomy removes the uterus for heavy bleeding, fibroids, prolapse or cancer. A pregnancy implanting outside the cavity, usually in the tube, is an ectopic pregnancy and a surgical emergency if it ruptures.

Structures in the atlas 6

The modelled pieces that make up the uterus & cervix in the 3D atlas. Each opens in the explorer.

• Body of uterus
• Cervix of uterus
• External os of uterus
• Fundus of uterus
• Internal os of uterus
• Isthmus of uterus

The female reproductive in brief

This layer brings the female body into an atlas whose reference model is male. Its pieces come from the united female model of the Human Reference Atlas (HRA), the reference built by the HuBMAP Consortium and the NIH from the Visible Human Female, scaled and placed so that the pubic symphysis, sacral promontory and hip bones of the female pelvis coincide with those of the BodyParts3D pelvis; the breasts sit on the reference chest wall at the level of the fourth to fifth rib. The uterus lies behind and above the bladder and in front of the rectum, tilted forward; the cervix opens into the top of the vagina, which runs down and back to the perineum; the uterine tubes (fallopian tubes) arch from the upper corners of the uterus out over the ovaries, which lie against the side walls of the pelvis; and the broad, round, cardinal and uterosacral ligaments and the suspensory and ovarian ligaments hold everything in place. The female bladder from the same model, with its trigone and neck, replaces the male bladder while the layer is shown, and a short schematic urethra runs from its neck down the front of the vagina to the vestibule (the HRA model has no urethra mesh). Each breast is shown as its body (fat and gland), the lobes of the mammary gland, the lactiferous ducts and sinuses, the suspensory ligaments, the areola with its tubercles and the nipple. Two things to know when reading it. The Visible Human Female was a woman of 59, and the whole layer is scaled to the male pelvis, so the uterus and ovaries are those of a woman past the menopause and smaller than the textbook figures on the organ pages. And because the reference skin is male, the breasts stand outside the skin overlay when both are shown. Switching this layer on hides the male reproductive organs, bladder and urethra so the two bodies are not drawn through each other; switching it off brings them back. The Female body preset shows the whole body with the female organs. The vulva and clitoris are not part of the HRA model and remain a written page.

All structures of the female reproductive