STEPTRACKERREPRODUCTIVE | PATHOLOGY + EMBRYOLOGY | HIGH-YIELD REVIEW

Myometrium & Uterine Structure

Fibroids versus their rare malignant cousin, the uterine sarcomas, and the Müllerian anomalies that shape the uterus.

FOCUS ON
Leiomyoma (fibroid): most common tumor in women; benign, estrogen-sensitive, multiple, whorled; heavy bleeding, pressure, infertility; grows in pregnancy, shrinks after menopause. Leiomyosarcoma: arises de novo, single, large, necrotic, many mitoses, postmenopausal. Müllerian anomalies: septate (recurrent miscarriage), bicornuate, didelphys, agenesis (MRKH).
LESION
MECHANISM
FEATURES
CLUE · TREATMENT
Leiomyoma
most common tumor in women
Benign clonal smooth-muscle tumor; MED12 mutations; estrogen and progesterone sensitive
Multiple, sharply circumscribed, white whorled nodules; more common and earlier in Black women
Do not become leiomyosarcoma
Leiomyoma — by location
Position decides symptoms
Submucosal: heavy bleeding, infertility, miscarriage · intramural: enlarged uterus, bleeding · subserosal: pressure (bladder frequency, constipation), pedunculated
Iron-deficiency anemia from menorrhagia
Leiomyoma — degeneration
Outgrows its blood supply
Hyaline (most common), cystic, calcific (postmenopausal), red degeneration (hemorrhagic infarct in pregnancy)
Acute pain + low fever in pregnancy
Leiomyoma — treatment
Reduce estrogen or remove tumor
GnRH agonist (continuous leuprolide) or antagonist shrinks fibroids; myomectomy (fertility), uterine artery embolization, hysterectomy
GnRH agonists are temporary (bone loss)
Leiomyosarcoma
Malignant smooth-muscle tumor arising de novo
Postmenopausal; single large soft mass with necrosis, hemorrhage, atypia, many mitoses
Rapidly enlarging "fibroid" after menopause · lung metastases
Endometrial stromal sarcoma
Tumor of endometrial stroma; often JAZF1 fusion (low grade)
Infiltrates myometrium and vessels ("worm-like" plugs)
Abnormal bleeding
Hereditary leiomyomatosis & RCC
Germline fumarate hydratase mutation (AD)
Cutaneous leiomyomas + early, multiple uterine fibroids + aggressive papillary RCC
Young woman with skin and uterine leiomyomas
Septate uterus
most common Müllerian anomaly
Incomplete resorption of the septum between fused paramesonephric ducts
Normal outer contour; recurrent pregnancy loss
Hysteroscopic resection
Bicornuate uterus
Incomplete fusion of the paramesonephric ducts
Heart-shaped uterus with indented fundus; malpresentation, preterm birth
Fusion vs resorption failure
Uterus didelphys
Complete failure of fusion
Double uterus, double cervix, often double vagina
Usually compatible with pregnancy
Müllerian agenesis (MRKH)
Paramesonephric ducts fail to develop
Primary amenorrhea with normal breasts, pubic hair and ovaries; absent uterus and upper vagina
Normal female karyotype · renal anomalies

Mechanism — hormones and fibroid growth

Estrogen + progesterone
→
Fibroid growthReproductive years, pregnancy
Menopause / GnRH agonist
→
↓ Estrogen
→
Shrinkage
Paramesonephric ductsFuse (uterus, cervix, upper vagina)
→
Septum resorbs
→
Failure → didelphys, bicornuate, septate

Leiomyoma vs leiomyosarcoma

FEATURE
LEIOMYOMA
LEIOMYOSARCOMA
Age
Reproductive
Postmenopausal
Number
Multiple
Single
Gross
Firm, white, whorled
Soft, hemorrhagic, necrotic
Histology
Bland, few mitoses
Atypia, many mitoses
Origin
Benign clone
De novo (not from fibroid)
After menopause
Shrinks
Grows

How it's tested

COMMON CONFUSIONBicornuate (failed fusion — dented outer fundus) vs septate (failed resorption — smooth outer fundus). Septate carries the higher miscarriage risk.
THE TRAPA "fibroid" that keeps growing after menopause should raise concern for leiomyosarcoma, not reassurance.
YOUR APPROACHUterine mass → age + number + growth with hormones → gross/microscopic features. Structural anomaly → fusion vs resorption vs agenesis → reproductive consequence.

If you see… think…

Heavy periods, iron deficiency, irregular enlarged uterus → leiomyomas
Whorled firm white uterine nodules → leiomyoma
Pregnant woman with acute pain over a known fibroid → red degeneration
Drug used to shrink fibroids before surgery → GnRH agonist (continuous leuprolide)
Rapidly growing, necrotic uterine mass at 60 → leiomyosarcoma
Skin leiomyomas + uterine fibroids + kidney cancer → fumarate hydratase deficiency
Recurrent miscarriages, smooth fundus, uterine septum → septate uterus
Heart-shaped uterus on imaging → bicornuate uterus
Two cervices and two uterine cavities → uterus didelphys
Primary amenorrhea, normal breasts, absent uterus, 46,XX → Müllerian agenesis (MRKH)