Also called inadequate luteal phase. This is a defect in which the lining of the uterus (endometrium) either is not exposed to enough progesterone or does not respond properly to the progesterone that is produced. If the corpus luteum, which produces progesterone, is not a good one and the production of progesterone is therefore less than adequate, then the uterine lining will become fragile before the time you'd expect a well-timed period. So an inadequate corpus luteum can be responsible for pre-period spotting. It can also be involved in miscarriage. With an inadequate luteal phase, the cells of the endometrium do not undergo the very orderly series of changes needed to allow implantation to occur. Consequently, even if a conception does occur, the endometrium may not be ready to accept it and, therefore, implantation will not occur. Progesterone is necessary to keep the lining of the uterus intact for the pregnancy, and if it's too low, a miscarriage can occur.