The Menstrual Cycle: Description of the cycle

Preparing for Implantation
Shortly before ovulation, the follicle that encases the ovum begins to produce a new hormone called progesterone. This production increases dramatically after ovulation when the ruptured follicle is transformed into a "yellow body" (corpus luteum). In addition to progesterone, which plays an important part in the final preparation of the uterine lining, the yellow body also continues the production of estrogen. Under the influence of this hormonal stimulation, the lining of the uterus soon achieves its greatest thickness and receptivity for implantation. After its release from the ovary, the ovum enters the nearest Fallopian tube and begins traveling toward the uterus. Within a few hours, it undergoes its final maturation and becomes ready for fertilization. After its fertilization, the ovum develops into a cell cluster which continues its journey through the Fallopian tube to the uterus, where it arrives about three days later. Finally, after another three to four days, it begins to implant in the nourishing uterine lining, thus starting pregnancy. A pregnancy helps to sustain the yellow body and its production of estrogen and progesterone. As a result, no new ovulation occurs, and the uterine lining does not deteriorate. In other words, in case of a pregnancy, the menstrual cycle is suspended at this point and does not enter the third phase described here.