A 28-year-old woman presents with amenorrhea for 3 months. Her last menstrual period was 12 weeks ago. Physical examination is unremarkable. Urine pregnancy test is negative. Which investigation is most appropriate to determine the phase of the menstrual cycle and assess ovulatory status?

Explanation

## Investigation of Choice for Assessing Ovulation **Key Point:** Serum progesterone in the luteal phase (day 21 of a 28-day cycle, or 7 days before expected menses) is the gold standard to confirm ovulation and assess luteal phase adequacy. ### Why Progesterone in Luteal Phase? 1. **Timing is critical:** Progesterone peaks 7–8 days after ovulation (mid-luteal phase). 2. **Threshold value:** A level >3 ng/mL (or >10 nmol/L) confirms ovulation has occurred. 3. **Single measurement suffices:** Unlike estradiol, which fluctuates throughout the cycle, a single mid-luteal progesterone level provides definitive evidence of corpus luteum function. ### Why Other Options Are Suboptimal | Investigation | Role | Limitation | |---|---|---| | Transvaginal ultrasound | Visualizes follicular development, corpus luteum | Operator-dependent; does not directly measure hormonal status | | Serum estradiol (day 3) | Assesses ovarian reserve | Measures follicular phase; does not confirm ovulation | | Endometrial biopsy | Evaluates endometrial receptivity | Invasive; rarely used for cycle assessment; does not confirm ovulation | **Clinical Pearl:** In this case, the patient has amenorrhea for 3 months with negative pregnancy test. Measuring progesterone in the expected luteal phase (if she has any cycle) or after a progestin challenge will clarify whether she is ovulating and help differentiate anovulation from other causes of amenorrhea. **High-Yield:** Progesterone >3 ng/mL = ovulation confirmed. Progesterone <1 ng/mL = anovulation or luteal phase defect.