A procedure that involves taking a small sample of tissue from the inside lining of the uterus (called the endometrium). An endometrial biopsy is done for many reasons. In a case of investigation for infertility, it is performed to evaluate the endometrium for its readiness to accept the embryo. An abnormal test is said to be "out-of-phase" in relation to the date of the cycle. An endometrial biopsy is also performed for abnormal uterine bleeding to diagnose hormonal imbalances or an anatomic cause for the bleeding, such as polyps, hyperplasia (abnormal benign growth of the endometrium) or cancer.