A 32-year-old woman presents with severe dysmenorrhea and chronic pelvic pain. Transvaginal ultrasound reveals multiple cystic lesions in the ovaries with echogenic fluid. What is the most common site of endometriosis in this patient?

Explanation

## Most Common Site of Endometriosis **Key Point:** The ovary is the most common site of endometriosis, accounting for approximately 35–40% of all endometrial implants. These ovarian lesions typically appear as chocolate cysts (endometriomas) on imaging. ### Frequency Distribution of Endometriosis Sites | Site | Frequency | Clinical Presentation | |------|-----------|----------------------| | Ovary | 35–40% | Chocolate cysts, dysmenorrhea, infertility | | Pouch of Douglas | 30–35% | Dyspareunia, dyschezia, pelvic pain | | Uterosacral ligament | 15–20% | Dysmenorrhea, dyspareunia | | Rectosigmoid colon | 10–15% | Dyschezia, rectal bleeding during menses | | Bladder | 5–10% | Dysuria, hematuria | | Other sites (peritoneum, bowel, lung) | <5% | Variable | **High-Yield:** The "chocolate cyst" (endometrioma) is pathognomonic for ovarian endometriosis. It contains old hemorrhagic material and appears as a well-defined cystic lesion with low-level echoes on ultrasound. **Clinical Pearl:** Ovarian endometriosis is strongly associated with infertility due to follicle dysfunction, altered follicular fluid composition, and adhesion formation. The presence of ovarian endometriomas increases the risk of epithelial ovarian cancer (though absolute risk remains low). ### Why Ovary Is Most Common 1. **Anatomical proximity** — ovaries are in direct contact with peritoneal fluid containing shed endometrial cells 2. **Retrograde menstruation** — menstrual debris preferentially deposits on ovarian surface 3. **Metaplasia theory** — coelomic epithelium of ovarian surface is susceptible to metaplastic transformation 4. **Implantation** — ovarian stroma provides a permissive microenvironment for ectopic endometrial growth **Mnemonic:** **POURED** — Pouch of Douglas, Ovary, Uterosacral ligament, Rectosigmoid, Endometrium (adenomyosis), Diaphragm — common sites of endometriosis in descending order of frequency (with ovary being second only to pouch of Douglas in some series, but most consistently reported as most common in modern cohorts). [cite:Berek & Novak's Gynecology 16e Ch 12]