A 32-year-old woman with a regular 28-day menstrual cycle reports heavy menstrual bleeding (menorrhagia) and dysmenorrhea. She wishes to understand the hormonal basis of her symptoms. Which investigation would best demonstrate the peak of the luteal phase and correlate with her symptoms?

## 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.