A 28-year-old woman presents with irregular menstrual cycles of 24–45 days duration. She reports normal secondary sexual characteristics and no galactorrhea. Physical examination is unremarkable. Which investigation is most appropriate to confirm the diagnosis of anovulation and assess ovarian reserve?

## Investigation of Choice for Anovulation Confirmation ### Why Serum Progesterone on Day 21 Is Correct **Key Point:** Serum progesterone measured in the luteal phase (day 21 of a 28-day cycle, or 7 days before expected menses) is the single most specific and practical test to confirm ovulation. A level >3 ng/mL (>9.5 nmol/L) indicates ovulation has occurred; levels <3 ng/mL suggest anovulation. **High-Yield:** Progesterone is secreted by the corpus luteum only after ovulation. It is the most direct biochemical marker of ovulation and requires no cycle regularity assumptions. **Clinical Pearl:** In irregular cycles, timing should be adjusted: measure 7 days before the expected next menses, or use a baseline day 3 FSH to estimate cycle length, then calculate day 21 equivalent. ### Why Other Investigations Are Not First-Line | Investigation | Limitation in This Context | |---|---| | **Transvaginal ultrasound** | Assesses ovarian morphology and follicle count (useful for PCOS, DOR), but does NOT directly confirm ovulation in this cycle. Requires expertise; not specific for anovulation diagnosis. | | **Day 3 FSH/LH** | Measures basal gonadotropins to assess ovarian reserve and hypothalamic–pituitary function, but does NOT confirm whether ovulation occurred in the current cycle. Useful for prognosis, not diagnosis of anovulation. | | **Endometrial biopsy** | Invasive, rarely indicated. Historically used for luteal phase defect (now discredited). No role in confirming anovulation. | ### Diagnostic Algorithm ```mermaid flowchart TD A[Irregular menses, normal exam]:::outcome --> B{Confirm ovulation?}:::decision B -->|Yes| C["Serum progesterone day 21<br/>(>3 ng/mL = ovulation)"]:::action C --> D{Progesterone level?}:::decision D -->|>3 ng/mL| E[Ovulatory cycle]:::outcome D -->|<3 ng/mL| F[Anovulation confirmed]:::outcome F --> G{Assess reserve & etiology?}:::decision G -->|Yes| H[Day 3 FSH, LH, TSH, Prolactin]:::action G -->|Yes| I[Transvaginal ultrasound]:::action ``` **Mnemonic:** **PCOS** = **P**rogesterone **C**onfirms **O**vulation **S**tatus. Check progesterone first; if low, then investigate cause with hormonal panel and imaging.