"In evaluating an endometrial biopsy specimen, an adequate clinical history is important, including the age of the patient and the reason for the biopsy. The menopausal status as well as the date of onset of the last menstrual period and the length of the menstrual cycle in premenopausal women should be provided. In many cases of postmenopausal bleeding, the patient is not actually postmenopausal but rather is perimenopausal, with a prolonged interval between periods. This results in the clinician and the patient assuming that the woman is postmenopausal. Before biopsy, many women with abnormal uterine bleeding are already taking exogenous hormones, especially progestogenic compounds, to control the bleeding, and this information is not always conveyed to the pathologist. Other women may be taking hormone replacement therapy or contraceptives. These hormonal compounds may alter the morphological appearance of the endometrium and a knowledge that these, and other relevant drugs such as tamoxifen, are being taken is of paramount importance to the pathologist."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1860448/