Regarding the structural and functional changes in the endometrium during the menstrual cycle, all of the following are true EXCEPT:

Explanation

## Endometrial Changes Across the Menstrual Cycle ### Proliferative (Follicular) Phase — Estrogen Dominance **Key Point:** Estrogen stimulates endometrial proliferation. Glandular epithelium undergoes mitosis, and endometrial thickness increases progressively from ~2 mm (post-menstrual) to 8–12 mm by the late follicular phase. **High-Yield:** Endometrial thickness peaks in the **late follicular phase** (just before ovulation), not during the luteal phase. This is a frequent exam trap. ### Secretory (Luteal) Phase — Progesterone Dominance **Key Point:** Progesterone transforms the endometrium into a secretory tissue: - **Subnuclear vacuoles** appear in glandular epithelium (day 18–22 of cycle), indicating active progesterone effect and preparation for implantation. - **Spiral arteries** elongate and become increasingly coiled (tortuous) in response to progesterone. - Edema increases in the stroma. - Glycogen accumulates in glandular cells. **Mnemonic:** **SSGE** — Secretory phase: Subnuclear vacuoles, Spiral artery coiling, Glycogen, Edema. ### Menstruation — Progesterone Withdrawal **Key Point:** Menstruation is NOT caused by vasoconstriction of spiral arteries. Rather, it is caused by **vasodilation** of spiral arteries following progesterone withdrawal, combined with increased prostaglandin production (especially PGF₂α), which causes: 1. Increased vascular permeability 2. Edema and tissue swelling 3. Rupture of spiral arteries 4. Ischemic necrosis of the superficial endometrium 5. Shedding of the functionalis layer **Clinical Pearl:** The mechanism of menstruation involves **vasodilation and increased prostaglandin production**, not vasoconstriction. Excessive prostaglandins cause dysmenorrhea; NSAIDs reduce prostaglandin synthesis and alleviate pain. **Warning:** A common misconception is that menstruation results from "ischemia due to vasoconstriction." The actual mechanism involves vasodilation, increased permeability, and prostaglandin-mediated inflammation. ### Comparative Table: Endometrial Changes | Phase | Thickness | Glandular Changes | Vascular Changes | Dominant Hormone | |-------|-----------|-------------------|------------------|------------------| | Early Proliferative | 2–3 mm | Mitosis, elongation | Minimal | Estrogen | | Late Proliferative | 8–12 mm | Continued proliferation | Minimal | Estrogen | | Early Secretory | 10–12 mm | Subnuclear vacuoles appear | Spiral artery coiling begins | Progesterone | | Mid-Secretory | 10–14 mm | Maximal vacuoles, glycogen | Marked coiling, edema | Progesterone | | Late Secretory | 10–14 mm | Vacuoles regress, predecidual changes | Maximal coiling, increased fragility | Progesterone (declining) | | Menstruation | <5 mm | Shedding of functionalis | Vasodilation, rupture, hemorrhage | Progesterone withdrawal | **High-Yield:** Endometrial thickness is **maximal in the late follicular and early-to-mid luteal phases** (10–14 mm), not progressively increasing throughout the cycle.