An endometrial biopsy taken on day 17 of a 28-day cycle shows subnuclear vacuolation in the glandular epithelium. What does this finding indicate?

• A Anovulatory cycle with persistent estrogen effect
• B Normal proliferative phase consistent with day 17
• C Luteal phase defect with out-of-phase endometrium
• D Ovulation has occurred approximately two days earlier, with early progesterone effect ✓

Explanation

Subnuclear vacuoles are the earliest reliable histological sign of ovulation, appearing about 36 to 48 hours after ovulation as progesterone drives glycogen accumulation above the nucleus before it shifts supranuclearly. Their presence on day 17 dates ovulation to around day 15. In an anovulatory cycle the endometrium remains proliferative, and a luteal phase defect would show a lag between chronological and histological dating rather than clear vacuolation.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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