21.1 Uterine Anatomy, Myometrium & Fibroids
Key Takeaways
- A normal nulliparous uterus measures roughly 6-8 cm in length and 3-5 cm in width and AP diameter; the multiparous uterus may reach about 10 cm in length
- Anteverted is the most common uterine position; a retroverted uterus tilts posteriorly as a unit, while a retroflexed uterus has the body bent backward on the cervix
- Leiomyomas (fibroids) are the most common benign pelvic tumor; the intramural location is most common, but submucosal fibroids cause the most abnormal bleeding
- Adenomyosis produces a diffusely enlarged, globular uterus with a heterogeneous myometrium, small myometrial cysts, an indistinct endomyometrial junction, and striated "Venetian blind" shadowing
- A well-circumscribed hypoechoic mass with shadowing favors a fibroid; an ill-defined, non-encapsulated heterogeneous area favors focal adenomyosis (adenomyoma)
Normal Uterine Anatomy and Positions
The uterus is a thick-walled, pear-shaped muscular organ that sits in the true pelvis between the bladder (anteriorly) and the rectum (posteriorly). It is divided into the fundus (the dome above the fallopian tube insertions), the body (corpus), the isthmus, and the cervix. In the adult, the body-to-cervix ratio is approximately 2:1, whereas in the prepubertal uterus the cervix is larger than the body (1:1 or even 2:1 cervix-to-body).
Normal adult measurements you should know for the exam:
- Nulliparous uterus: approximately 6-8 cm long, 3-5 cm wide, 3-5 cm in anteroposterior (AP) diameter
- Multiparous uterus: each pregnancy enlarges the uterus slightly; length may reach about 10 cm
- Postmenopausal uterus: atrophies without estrogen stimulation and may shrink to 3-5 cm in length
Uterine position describes how the uterus is angled relative to the vagina and the cervix, and it directly affects how you orient the transducer and label images.
| Position | Description | Exam Note |
|---|---|---|
| Anteverted | Entire uterus tilted forward toward the bladder | The most common position |
| Anteflexed | Body bent forward on the cervix | Often coexists with anteversion |
| Retroverted | Entire uterus tilted posteriorly as a unit | Fundus points toward the sacrum |
| Retroflexed | Body bent backward on the cervix | Fundus can be hard to see transabdominally; transvaginal imaging helps |
Version describes the axis of the whole uterus relative to the vagina; flexion describes the bend of the body relative to the cervix. A uterus can therefore be retroverted and retroflexed at the same time.
The Myometrium
The myometrium is the smooth-muscle wall of the uterus and has three functional layers, though you will not always see them separately on ultrasound:
- Outer (longitudinal) layer — thin; its fibers run along the long axis of the uterus
- Middle (spiral/interlacing) layer — by far the thickest; contains the bulk of the muscle mass and the larger vessels. The arcuate arteries course circumferentially in the outer third of the myometrium and give off radial branches inward; arcuate artery mineralization can be seen as peripheral echogenic foci in older patients
- Inner (compact) layer — also called the junctional zone; it appears as a thin, hypoechoic subendometrial halo immediately surrounding the endometrium
The junctional zone matters clinically: a markedly thickened or indistinct junctional zone (about 12 mm or more) is one of the supporting signs of adenomyosis.
Leiomyomas (Fibroids)
A leiomyoma, commonly called a fibroid or myoma, is a benign, monoclonal tumor of smooth muscle and is the most common pelvic tumor in women — present in roughly 20-30% of women over 30 and in a majority of women by age 50. Fibroids are estrogen responsive: they tend to grow during pregnancy and with estrogen stimulation and regress after menopause. A rapidly enlarging "fibroid" in a postmenopausal patient is therefore a red flag.
Locations
- Intramural — within the myometrial wall; the most common location
- Subserosal — projects from the outer (serosal) surface; may become pedunculated and mimic an adnexal mass
- Submucosal — beneath the endometrium, distorting the cavity; the least common but the most symptomatic, classically causing menorrhagia and infertility
- Cervical or broad-ligament fibroids are less common variants
The FIGO classification (types 0-8) formalizes this: types 0-2 are submucosal (0 = pedunculated intracavitary), types 3-5 are intramural or "other" (3 abuts the endometrium without distorting it; 5 is subserosal but at least half intramural), types 6-7 are subserosal (7 = pedunculated subserosal), and type 8 covers cervical, parasitic, and broad-ligament fibroids. You do not need every number, but you should know the concept that classification is based on the relationship of the fibroid to the endometrium and serosa.
Sonographic Appearance
The classic fibroid is a well-circumscribed, hypoechoic, solid mass with a whorled internal architecture that causes posterior acoustic attenuation or shadowing and distorts the uterine contour. Peripheral (rim) vascularity is typical on color Doppler. Fibroids frequently undergo degeneration as they outgrow their blood supply:
- Hyaline degeneration — the most common type; increased heterogeneity
- Cystic degeneration — anechoic spaces within the mass
- Red (carneous) degeneration — hemorrhagic infarction, classically during pregnancy, causing acute pain
- Calcific degeneration — coarse, rim-like "popcorn" calcifications with dirty shadowing, typical of postmenopausal fibroids
Adenomyosis
Adenomyosis is the presence of ectopic endometrial glands and stroma within the myometrium, causing surrounding smooth-muscle hypertrophy. It typically affects parous women in their 40s and presents with dysmenorrhea and heavy menstrual bleeding. Transvaginal sonographic features include:
- Diffusely enlarged, globular (rounded) uterus
- Asymmetric myometrial thickening, usually of the posterior wall
- Heterogeneous myometrium with a striated, "Swiss cheese" texture
- Small (about 2-6 mm) myometrial cysts — tiny anechoic lakes within the wall
- Indistinct endomyometrial junction with a thickened junctional zone
- "Venetian blind" (rain-shower) shadowing — fine linear striations fanning through the myometrium
Fibroid vs. Focal Adenomyosis (Adenomyoma)
| Feature | Fibroid | Focal adenomyosis |
|---|---|---|
| Margins | Well circumscribed, pseudocapsule | Ill defined, no capsule |
| Echogenicity | Hypoechoic, whorled | Heterogeneous, striated |
| Shadowing | Broad attenuation | Fine Venetian-blind striations |
| Mass effect | Distorts endometrium/contour | Little mass effect |
| Vascularity | Peripheral rim | More central/diffuse |
Uterine Sarcoma
Leiomyosarcoma and other uterine sarcomas are rare (a small fraction of uterine malignancies) but aggressive. Ultrasound cannot reliably distinguish a sarcoma from a degenerating fibroid. Features that raise suspicion include rapid growth, enlargement after menopause, a large heterogeneous mass with cystic necrosis, and prominent chaotic internal vascularity. Definitive diagnosis is histologic, usually after surgery.
A 38-year-old patient presents with heavy menstrual bleeding. Transvaginal ultrasound shows a well-circumscribed hypoechoic mass that indents and distorts the endometrial cavity from beneath it. Which fibroid location best explains her bleeding symptoms?
Which sonographic feature is most characteristic of adenomyosis?
When trying to distinguish focal adenomyosis (adenomyoma) from a fibroid, which finding favors an adenomyoma?