Progesterone, made by the corpus luteum (ruptured follicle) after the egg is released, stimulates the glands in the endometrium to begin secreting their nourishing substance. This is the secretory phase of the cycle and is the only time when a fertilized egg can implant in the lining. In the case of irregular periods, it is the proliferative phase that is variable; for example, if you have 28-day cycles, you ovulates on day 14; if you have 35-day cycles, you ovulate on day 21.