Luteal phase deficiency (LPD) case. In cycle 1, this woman menstruated from a day 25 endometrium, which is 3 days premature and out-of-phase and compatible with LPD. In cycle 2, her menstrual period occurred 12 days after a biopsy that was read as histologic day 24. The most likely explanation for this prolonged luteal phase is an early spontaneous abortion (chemical pregnancy). In the third cycle, the patient bled from a day 24 endometrium, which is 4 days out-of-phase and confirms the diagnosis of LPD. Bx, biopsy; NMP, next menstrual period.