A 28-year-old woman with a 3-year history of primary infertility presents for evaluation. She has regular 28-day menstrual cycles and normal pelvic examination. Her partner's semen analysis is normal. Before initiating ovulation induction therapy, which investigation is most appropriate to confirm ovulatory status?

## Investigation of Choice for Confirming Ovulation **Key Point:** Serum progesterone measurement in the luteal phase (7 days before expected menses, typically day 21 of a 28-day cycle) is the gold standard investigation to confirm ovulation. ### Mechanism of Confirmation Progesterone is produced by the corpus luteum only after ovulation has occurred. A level >3 ng/mL (or >10 nmol/L) indicates ovulation; levels >10 ng/mL confirm adequate luteal phase function. ### Why This Patient Needs Confirmation Although the patient has regular cycles (suggesting ovulation), confirmation is essential before starting ovulation induction therapy because: - Regular cycles do NOT always indicate ovulation (anovulation can occur with regular bleeding) - Baseline ovulatory status guides the choice of agent and dosing - Establishes whether the infertility is truly due to anovulation or another factor ### Timing and Interpretation | Progesterone Level | Interpretation | |---|---| | <3 ng/mL | Anovulation (no ovulation) | | 3–10 ng/mL | Borderline; may indicate inadequate luteal phase | | >10 ng/mL | Confirmed ovulation with adequate luteal phase | **Clinical Pearl:** A single progesterone level is sufficient for diagnostic confirmation; serial measurements are not required for diagnosis but may be used to assess luteal phase adequacy in recurrent pregnancy loss. **High-Yield:** The test must be timed correctly — 7 days before expected menses. Mistiming the sample (e.g., taking it too early) can give falsely low results and lead to incorrect diagnosis of anovulation.