## Investigation to Assess Luteal Phase Hormone Status **Key Point:** Serum progesterone measured on day 21 of a 28-day cycle (7 days post-ovulation) reflects peak luteal phase progesterone and correlates with endometrial changes that cause menorrhagia and dysmenorrhea. ### Hormonal Basis of Menorrhagia and Dysmenorrhea 1. **Progesterone in luteal phase:** Peaks on day 21, inducing endometrial secretory changes. 2. **Prostaglandin production:** High progesterone stimulates endometrial prostaglandin synthesis (especially PGF~2α~), causing uterine contractions and dysmenorrhea. 3. **Endometrial thickness:** Progesterone promotes endometrial proliferation and vascularity, contributing to heavy bleeding. ### Timing of Hormonal Peaks in a 28-Day Cycle | Day | Event | Hormone Peak | |---|---|---| | 1–5 | Menstruation | Low estradiol, low progesterone | | 14 | Ovulation | LH surge | | 21 | Mid-luteal phase | **Progesterone peak** | | 28 | Late luteal phase | Progesterone decline | **Clinical Pearl:** Dysmenorrhea and menorrhagia worsen in the luteal phase when progesterone is highest and endometrial prostaglandin production is maximal. Measuring progesterone on day 21 directly reflects this pathophysiology. **High-Yield:** Day 21 progesterone >10 ng/mL = adequate luteal phase; correlates with secretory endometrium and prostaglandin-driven symptoms. **Mnemonic:** **MLP** = Mid-Luteal Progesterone (day 21) — the gold standard for luteal phase assessment. 
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