A 32-year-old woman presents with primary infertility of 3 years duration. Menarche at age 13, cycles regular (28 days), duration 4–5 days. BMI 24 kg/m². General examination and pelvic examination are normal. Husband's semen analysis is normal. Day 21 serum progesterone is 8 ng/mL (normal luteal phase >10 ng/mL). What is the most appropriate next step in management?