Uterus Anatomy

Uterus

The uterus is a hollow, thick-walled, muscular organ of the female reproductive tract that lies in the lesser pelvis. The body of the uterus is intraperitoneal and the cervix uteri is subperitoneal.

Gross anatomy

The uterus has an inverted pear shape. It measures about 7.5 cm in length, 5 cm wide at its upper part, and nearly 2.5 cm in thickness in adults. It weighs approximately 30-40 g.

The uterus is divisible into two portions: body and cervix. About midway between the apex and base is a slight constriction known as the isthmus. The portion above the isthmus is termed the body, and that below, the cervix. The part of the body which lies above a plane passing through the points of the entrance of the uterine tubes is known as the fundus.

The body gradually narrows from the fundus to the isthmus. The cavity of the body is a mere slit, flattened anteroposteriorly. It is triangular in shape:

• the base being formed by the internal surface of the fundus between the orifices of the uterine tubes
• the apex by the internal orifice of the uterus through which the cavity of the body communicates with the canal of the cervix

Although anatomically a part of the uterus, the uterine cervix has a different function and is associated with separate pathological entities. It is discussed in detail in a separate article.

Attachments

Musculotendinous and ligamentous

• anterior: pubocervical ligament
• lateral: transverse cervical ligaments (cardinal or Mackenrodt's)
• posterior: uterosacral ligaments
• inferior: puborectalis and pubovaginalis parts of the levator ani muscle

Relations

• anteriorly: bladder; uterovesical pouch
• posteriorly: rectum; pouch of Douglas
• laterally: broad ligament; round ligament; uterine vessels
• uterine tubes open into its upper part
• inferiorly: uterine cavity communicates with that of the vagina

Position

Uterine position in adults is variable and largely depends on the degree of bladder and rectal distension. When the bladder is empty, the entire uterus is directed forward and is at the same time bent on itself at the junction of the body and cervix so that the body lies upon the bladder. As the latter fills, the uterus gradually becomes more and more erect until, with a fully distended bladder, the fundus may be directed back toward the sacrum.

The uterine position can be described in terms of version and flexion:

• uterine version: defined as the angle that the cervical axis makes with the vaginal axis 8
• anteversion
• version angle <90° 8
• external os points posteriorly towards the rectum 9
• retroversion
• version angle >90° 8
• external os points superiorly towards the bladder 9
• axial: external os points towards the introitus 9
• uterine flexion: defined as the angle that the uterine body axis makes with the cervical axis 8
• anteflexion: flexion angle <180° and the apex is directed anteriorly 8
• retroflexion: flexion angle >180° and apex is directed posteriorly 8

This results in a number of uterine positions:

• anteverted anteflexed: cervix angles forward, body is flexed forward

Development

In the fetus, the uterus is contained in the abdominal cavity, projecting beyond the superior aperture of the pelvis. The cervix is considerably larger than the body.

At puberty, the uterus is pyriform in shape and weighs from 14-17 g. It has descended into the pelvis, the fundus being just below the level of the superior aperture of this cavity. The palmate folds are distinct and extend to the upper part of the cavity of the organ.

During menstruation, the uterus is enlarged, more vascular, and its surfaces rounder; the external orifice is rounded, its labia are swollen, and the lining membrane of the body thickened, softer, and of a darker color.

During pregnancy, the uterus becomes enormously enlarged, and by the eighth month, it reaches the epigastric region. The increase in size is partly due to the growth of pre-existing muscles and partly to the development of new fibers.

After parturition, the uterus nearly regains its usual size, weighing about 42 g. Its cavity is larger than in the pre-pregnancy state, its vessels are tortuous, and its muscular layers are more defined. The external orifice is more marked, and its edges present one or more fissures.

In old age, the uterus becomes atrophied and paler and denser in texture; a more distinct constriction separates the body and cervix. The internal orifice is frequently, and the external orifice is occasionally obliterated, while the lips almost entirely disappear.

Arterial supply

• uterine arteries and ovarian arteries
• the terminations of the ovarian and uterine arteries unite and form an anastomotic trunk from which branches are given off to supply the uterus
• in the gravid uterus, the arteries carry the blood to the intervillous space of the placenta

Venous drainage

• uterine vein draining into internal iliac vein
• in the gravid uterus, the veins convey blood away from the intervillous space of the placenta

Lymphatic drainage

• fundus: para-aortic nodes
• body/cervix: internal and external iliac nodes; superficial inguinal nodes (via round ligament) 2

Innervation

The nerves are derived from the inferior hypogastric and ovarian plexuses (via the aorticorenal plexuses) from the third and fourth sacral nerves.

Variant anatomy

• see Müllerian duct anomalies

Radiographic features

Ultrasound is the primary diagnostic modality in characterizing uterine lesions. Meanwhile, CT scan is normally done for other non-gynecological indications. If there is indeterminate lesion found on ultrasound or CT, MRI is helpful in characterizing the uterine lesions 7.

Fluoroscopy

• see: hysterosalpingogram (HSG)

Ultrasound

The transabdominal US allows evaluation of the size and position of the uterus in the pelvic cavity. The transvaginal US allows the internal structure of the uterus to be examined 2.

CT

The normal uterus lies between the bladder and rectum and myometrial attenuation is between 30-50 HU depending on age and hormonal influences. The endometrial complex is relatively hypoattenuating to myometrium and this difference persists following IV contrast medium injection. The myometrium typically enhances earlier and more intensely 2. The enhancement pattern is variable 7:

• type 1: thick or thin sub-endometrial enhancement, most commonly found in pre-menopausal women
• type 2: diffuse myometrial enhancement, found in both pre and post-menopausal women
• type 3: faint diffuse myometrial enhancement, exclusively found in post-menopausal women
• patchy myometrial enhancement

These variable enhancement patterns could be due to differences in age, parity and menstrual status. The basal layer of endometrium adjacent to the myometrium could contribute to thin sub-endometrial enhancement. Thick sub-endometrial enhancement could be due to the junctional zone. Meanwhile, diffuse myometrial enhancement could be due to the density of smooth muscles and blood vessels in the myometrium that affects the uptake of contrast 7.

The cervix is relatively hypoattenuating and may have a target appearance after IV contrast medium with a central ring of lower enhancement. Enhancement is relatively delayed, compatible with the fibrous nature of the cervix.

MRI

MRI displays the zonal anatomy of the uterus. The myometrial layers are indistinguishable on T1 imaging. It can be divided into three zones on T2 weighted imaging 7:

• high T2 signal of endometrium
• low T2 signal of inner myometrium, known as the junctional zone
• intermediate T2 signal of the outer myometrium

On contrasted MRI study, myometrium shows homogeneous hyperintensity on arterial and venous phases. Meanwhile, endometrium enhances slowly and is more hypointense when compared to myometrium 7.

There is some physiologic variability to the myometrial zonal appearance. The junctional zone is less distinct pre-menarche and during pregnancy 4. In the postmenopausal patient, the outer myometrium is thinner and of lower signal due to reduced fluid content and therefore approximates the junctional zone, with poor delineation of the margin in some patients.

Myometrial zonal anatomy has diagnostic implications in the assessment of adenomyosis and in the staging of endometrial carcinoma, where the depth of myoinvasion is assessed in relation to the junctional zone.

Endometrium on MRI undergoes expected physiological variation in thickness, but the structural, cyclical changes seen on ultrasound are not reproduced.

Imaging assessment of the uterine size

The uterine length is measured in the midsagittal plane from the outer serosal surface of the fundus to the external cervical os. The uterine height (AP) is measured in the midsagittal plane anteroposteriorly from outer to outer serosal surfaces. The uterine width is measured in the transverse plane. The uterine body length is measured in the midsagittal plane from the fundal outer serosal surface to the internal os. The uterine cervix length is measured in the midsagittal plane from the internal os to the external os.

Several methods in the assessment of the uterine size are present in the literature:

• Uterine volume measurement 5: using the formula; Length x Width x Height x 0.523

According to this method, normal mean uterine volumes are as follows:

Uterine length and uterine body to cervix ratio by life stage 6 :

Fallopian tubes

The fallopian tube (TA: tuba uterina 8), also known as the uterine tube or, less commonly, the oviduct, is a paired hollow tube that bridges the ovary and uterus and functions to convey the mature ovum from the former to the latter. If conception occurs, it usually does so within the tube, which can be affected by a wide range of pathologies.

Gross anatomy

The fallopian tube is approximately 10-12 cm long and 1-4 mm in diameter. It courses laterally between the ovary and the uterus. Through it, the ovum passes into the uterine cavity. The peritoneal reflection draping over the salpinges forms the mesosalpinx.

The fallopian tube can be divided into five anatomic segments (from lateral to medial, the path that an ovum takes after ovulation):

• fimbriae: ~25 finger-like projections that drape over the ovary; ovarian fimbriae are longer than the others and are attached to the tubal pole of the ovary
• infundibulum: a funnel-shaped lateral part that drapes over the ovary with the fimbriae emanating from it; opens into the peritoneal cavity at the abdominal ostium
• ampulla: the widest and longest section, forming over half the length, it is the most established location of fertilisation
• isthmus: immediately lateral to the uterus, it is the narrowest segment, as its name suggests
• interstitial or intramural segment: the section within the myometrium

Relations

The extrauterine part of the fallopian tube courses between the two folds of the broad ligament at its superior aspect 5.

• at its medial end, the interstitial segment is continuous with the uterotubal junction, a continuation of the uterine cavity
• at its lateral end, the infundibulum opens into the peritoneal cavity

The fallopian tubes are one of the components of the adnexa.

Arterial supply

• tubal branch of the ovarian artery (lateral one-third) and terminal (tubal) branch of the uterine artery (medial two-thirds)

Venous drainage

• lateral one-third via the pampiniform plexus to the ovarian veins
• medial two-thirds via the uterine plexus to the internal iliac vein

Lymphatic drainage

• via ovarian vessels to the para-aortic nodes and uterine vessels to the internal iliac chain
• some drainage also to the inguinal nodes via the round ligament

Innervation

• autonomic supply from the ovarian and uterine plexuses
• parasympathetic: vagus for the lateral half, pelvic splanchnic nerves to the medial half
• sympathetic: thoracic and lumbar spinal segments (T10- L1)

Variant anatomy

• absent unilateral
• hypoplastic unilateral
• hypoplastic bilateral
• abnormal entry into uterine body or fundus

Broad ligaments

The broad ligaments (TA: ligamentum latum uteri or plica lata uteri 4) are the paired lateral folds of the parietal peritoneum which reflect over the upper genital tract in females.

Gross anatomy

The broad ligament extends from the lateral aspect of the uterus to the lateral pelvic wall and can be divided into three main components - the mesosalpinx, mesovarium and mesometrium. The broad ligament is usually considered to form part of the adnexa.

Attachments

• mesosalpinx
• superiorly: uterine tube
• superolaterally: suspensory ligament of the ovary
• posteroinferiorly: mesovarium
• medially: ovarian ligament
• mesometrium: drape-like largest portion of the broad ligament
• extends from the pelvic floor to the uterine body and ovarian ligament

Contents

The contents of the broad ligament include:

• adnexa
• ovary
• uterine tube
• blood vessels
• ovarian artery
• uterine artery
• ligaments
• ovarian ligament
• suspensory ligament of the ovary
• round ligament of the uterus
• other
• vestigial remnants of the mesonephric tubules
• a fraction of ureters 3

A useful mnemonic to remember the contents can be found here.

Variant anatomy

• congenital absence of the broad ligament

Related pathology

• broad ligament hernia
• common site for endometriomas
• benign tumors: usually lipoleiomyomas which are a variant of uterine leiomyomas
• malignant tumors (rare): leiomyosarcomas

Round ligament

The round ligament is one of the supporting structures of the uterus. It has a function in uterine anteflexion.

Embryology

Together with the ligament of the ovary, it is one of the remnants of the gubernaculum in females.

Gross anatomy

The round ligament is a rope-like fibromuscular band that extends from the anterolateral aspect of the uterus in the region of the cornu, just below the origin of the uterine tube. It courses anterolaterally between the layers of the broad ligament to the pelvic sidewall. It then passes through the deep inguinal ring into the inguinal canal. After exiting the canal through the superficial inguinal ring, it terminates in the labia majora and mons pubis.

Relations

• broad ligament
• ovarian ligament: also attaches at uterine cornu

Arterial supply

A branch of the inferior epigastric artery, the artery of the round ligament (also known as Sampson's artery) runs under the round ligament and provides an anastomosis between ovarian and uterine arteries.

Radiographic features

CT

Can be seen as a soft tissue density band between the uterus and deep inguinal ring.

MRI

The ligament is low T1 and T2 signal.

Related pathology

• endometriosis
• round ligament fibroid
• hydrocele of the canal of Nuck

Cardinal ligaments

The cardinal ligaments, also known as transverse cervical ligaments or Mackenrodt ligaments, are paired structures that act to support the pelvic organs of the female pelvis. These along with the uterosacral and pubocervical ligaments, provide support to prevent pelvic organ prolapse 1.

Gross anatomy

The cardinal ligaments originate at the border of the greater sciatic foramen before “inserting” at the lateral aspect of the superior vagina and cervix 1. Along with the uterosacral ligaments, they act to suspend the uterus, cervix, and upper vagina. This can be demonstrated by the cardinal ligaments being in a relatively vertical orientation when an individual is standing upright - resisting the effect of gravity on the structures it supports 1,2.

While referred to as a "ligament", the cardinal ligament is rather a condensation of connective tissue rather than a discrete ligamentous structure in the traditional sense 1,2.

The cardinal ligament consists primarily of vascular, neural, and areolar tissue containing fat with a network of connective tissue separating the different elements 2.

They are comprised of both a vascular component (located cranially) and a neural component (located caudally) 2,4:

• vascular component: contains the internal iliac artery, the uterine artery and uterine vein, smooth muscle, vaginal and vesical arteries, connective tissue, and lymph nodes 4
• neural component: contains nerve fibers of the hypogastric nerve and inferior hypogastric plexus and associated vessels 4

The ureter crosses the cardinal ligament at the point where the ligament passes inferior to the uterine vessels 4.

Radiographic features

Cardinal ligaments are best demonstrated on coronal T2-weighted MRI imaging. Vessels appear as dot-shaped structures with low T2 signal intensity, while areas of high T2 signal intensity correlate with adipose and connective tissue 3.

Clinical importance

Given the position of the uterine vessels and ureter relative to the cardinal ligament, manipulation of the structure during pelvic surgery can result in ureteric or uterine artery injury 4.

• Should the cardinal ligament become lax, the vaginal apex may position directly above the levator hiatus leading to an increased risk of prolapse due to the loss of suspensory support

Uterosacral ligaments

The paired uterosacral ligaments are one of the supporting structures of the uterus. Uterosacral ligaments are not infrequently affected in cases of deep infiltrating endometriosis.

Gross anatomy

The paired uterosacral ligaments are extraperitoneal structures which extend posteriorly from the uterine cervix to sacrum, forming the lateral boundaries of the rectouterine and rectovaginal spaces.

They are composed of smooth muscle and connective tissue along with splanchnic nerve fibers.

Attachments

• anterior: lateral aspect of the cervix and upper vaginal wall, torus uterinus
• posterior: presacral soft tissues between S2 and S4, including coccygeus muscle, sacrospinous ligament, ischial spine, and presacral fascia ​2

Radiographic features

Can be detected on CT as soft tissue attenuation bands, and on MRI as low T1 and T2 signal bands between the cervix and sacrum.

Related pathology

• endometriosis
• pelvic organ prolapse