A small sample (or biopsy) is taken of the lining of the uterus (the endometrium) just before a period is expected, aiming to show advanced stages of progesterone's effect on it, in the form of an adequate predecidual reaction. This a very sensitive test of the adequacy of the luteal phase. The biopsy can be combined with a laparoscopy or carried out in isolation as an office procedure. It's best to be sure that there is no possibility of pregnancy during the cycle the test is conducted, as occasionally the biopsy jeopardises an early pregnancy (namely an early implantation, before the pregnancy test or serum hCG has become positive).