Uterus

Uterus (metra)

Also known as: womb

A thick-walled, pear-shaped muscular organ in the lesser pelvis (≈7.5 × 5 × 2.5 cm) in which the embryo implants and the fetus develops; its endometrium cycles monthly and its myometrium expels the fetus at birth.

Show on body map Female Reproductive Organs Hypothalamic–Pituitary Axis

Overview

Size (nulliparous)≈7.5 × 5 × 2.5 cm

Mass≈30–60 g non-pregnant; ≈1 kg at term

Usual positionAnteverted and anteflexed (retroverted in ≈20 %)

The uterus has a domed fundus above the entry of the uterine tubes, a body (corpus) forming the upper two-thirds, a narrow isthmus (≈1 cm, which becomes the lower uterine segment in late pregnancy — the site of caesarean incisions), and the cervix (lower third), which projects into the vagina. The cavity of the body is a narrow triangular slit in coronal section, with the tubal openings at its upper angles and the internal os at its apex.

Position: typically anteverted (the cervix is tipped forward relative to the vagina, ≈90°) and anteflexed (the body is bent forward on the cervix), so the uterus lies almost horizontal on the bladder; about 1 in 5 women have a retroverted uterus. Peritoneum covers the fundus and body, reflecting anteriorly onto the bladder (vesicouterine pouch) and posteriorly over the posterior fornix onto the rectum (rectouterine pouch of Douglas, the lowest part of the female peritoneal cavity).

Wall: endometrium (mucosa), myometrium (thick smooth muscle) and perimetrium (serosa). Supports: the principal support is the pelvic floor (levator ani) and perineal body; the cardinal (transverse cervical), uterosacral and pubocervical ligaments — condensations of endopelvic fascia attached to the cervix and upper vagina — are the main ligamentous supports. The broad and round ligaments contribute little support (the round ligaments help maintain anteversion).

Location: Lesser pelvis, between the bladder (anteriorly) and the rectum (posteriorly), within the broad ligaments; in pregnancy it rises into the abdomen (fundus at the umbilicus at ≈20 weeks, at the xiphisternum at ≈36 weeks).

Parts & subdivisions

Fundus
Rounded upper part above the tubal openings; fundal height is measured in pregnancy.
Body (corpus) and cornua
Main part enclosing the uterine cavity; cornua where the tubes enter.
Isthmus
Narrow junction with the cervix; forms the lower uterine segment in late pregnancy.
Cervix
Lower cylindrical part with supravaginal and vaginal portions.
Endometrium
Cyclically shed mucosa.
Myometrium
Thick smooth-muscle layer.
Perimetrium
Peritoneal covering.

Anatomy

Innervation: Uterovaginal plexus (from the inferior hypogastric plexus): sympathetic fibres (≈T10–L1) and parasympathetic fibres (pelvic splanchnic, S2–S4). Visceral pain from the body/fundus (above the pelvic pain line) travels with sympathetic fibres to ≈T10–L1; pain from the cervix travels with parasympathetic fibres to S2–S4 — relevant to labour analgesia.
Arterial supply: Uterine artery (anterior division of the internal iliac) runs medially in the base of the broad ligament (with the cardinal ligament), crosses superior to the ureter ≈2 cm lateral to the supravaginal cervix, then ascends tortuously along the side of the uterus and anastomoses with the ovarian artery. In the wall: arcuate arteries (middle myometrium) → radial arteries → straight (basal) arteries to the stratum basalis and spiral arteries to the stratum functionalis.
Venous drainage: Uterine venous plexus in the broad ligament → uterine veins → internal iliac veins; communicates with the vaginal and ovarian plexuses.
Lymphatic drainage: Fundus and upper body → para-aortic nodes (with ovarian vessels); region near the uterotubal junction → along the round ligament to superficial inguinal nodes; body → external iliac nodes; cervix → internal iliac, external iliac, obturator and sacral nodes.
Relations: Anterior: vesicouterine pouch and bladder. Posterior: rectouterine pouch, sigmoid colon and rectum. Lateral: broad ligaments containing the uterine tubes, ovarian and round ligaments and uterine vessels; the ureter passes under the uterine artery beside the cervix. Inferior: vagina.
Development: Fusion of the caudal parts of the paramesonephric (Müllerian) ducts forms the uterovaginal primordium; the midline septum resorbs by ≈20 weeks.

Function

Receives the blastocyst and supports implantation and placentation.
Accommodates the growing fetus (enlarging from ≈50 g to ≈1 kg and ≈5 L capacity).
Expels the fetus and placenta at parturition and contracts to control bleeding afterwards.
Sheds the endometrium cyclically (menstruation) when pregnancy does not occur.

Clinical significance

Uterine fibroids and adenomyosis

Benign smooth-muscle tumours (leiomyomas) cause heavy bleeding, pressure symptoms and subfertility; adenomyosis (endometrium within the myometrium) causes a bulky tender uterus with dysmenorrhoea and heavy bleeding.

Endometrial cancer

Commonest gynaecological cancer in high-income countries; typically presents with postmenopausal bleeding. Risk factors relate to unopposed estrogen (obesity, PCOS, nulliparity, tamoxifen, estrogen-only HRT) and Lynch syndrome.

Uterine prolapse

Weakness of the pelvic floor and cardinal/uterosacral ligaments (vaginal childbirth, menopause, chronic straining) lets the uterus descend into the vagina, often with cystocele or rectocele.

Congenital uterine anomalies

Septate (commonest; recurrent miscarriage), bicornuate, didelphys and unicornuate uteri result from defects of Müllerian fusion or septal resorption; Müllerian agenesis (MRKH) causes primary amenorrhoea with normal ovaries.

Hysterectomy and the ureter

The ureter lies just below the uterine artery beside the cervix ("water under the bridge") and is at risk when the uterine vessels are clamped.

Test yourself

The round ligament of the uterus ends in the:

Sacrum
Labium majus, after passing through the inguinal canal
Lateral pelvic wall over the external iliac vessels
Ovary

Answer: Labium majus, after passing through the inguinal canal

The round ligament runs from the uterotubal junction through the deep inguinal ring and inguinal canal to the subcutaneous tissue of the labium majus. Like the ovarian ligament, it is a remnant of the gubernaculum.