An ovarian cycle has 2 phases: follicular and luteal. During the follicular phase, a follicle grows until it becomes mature enough and can ovulate. During ovulation, an egg-cell is expelled from the follicle and enters the Fallopian tube. The remaining follicle is transformed into a yellow body (corpus luteum). The follicular phase ends and the luteal phase begins with ovulation. A yellow body produces the hormone "progesterone", which is necessary for preparing the endometrium for implantation if the egg-cell becomes fertilized. The yellow body has a restricted lifetime. If there is no conception and implantation, it lives for about 14 days. After that time, it stops producing progesterone and transforms into a white body (corpus albicans). Lack of progesterone and estrogen then causes bleeding and desquamation of the functional endometrium (menstruation). But if conception and implantation occur, the embryo produces human chorion gonadotropin (hCG). hCG stimulates the corpus luteum to function longer (2-3 months) . This is called the yellow body of pregnancy (corpus luteum gravidarum).
The luteal phase is constant in duration (about 14 days) because the yellow body lives during such time. The duration of the whole cycle can vary in length due to variations in the follicular phase. Because the luteal phase is constant, we can predict an ovulation date (about 14 days before the next period is due). This prediction is possible only in women with regular cycles.