UNDERSTANDING THE MENSTRUAL CYCLE
A woman's ovaries produce the sex hormones progesterone and estrogen. These essential hormones not only contribute to sexual development but also prepare the uterus each month to both house and provide nourishment for an egg, should it become fertilized. So, each menstrual cycle, then, begins with the production of a few eggs in the ovaries. After a maturation process of about two to three weeks, one—on occasion more—of these eggs will be released and may be picked up by the fallopian tubes. This release is called ovulation. Once inside the fallopian tubes the now-mature egg is ready to be fertilized. Fertilization only occurs if a male's sperm finds its way to the waiting egg and is able to penetrate its wall. Should fertilization not happen, the egg(s) will degenerate (die). Two weeks later menstruation (also known as a period) occurs.
The entire process is governed by two sets of phases, the first of which consists of three phases: the follicular, ovulation, and luteal stages. During the follicular phase, Follical-Stimulating Hormone (FSH) levels increase, causing five to seven eggs that have been developing for approximately two and one half month to compete with one another for dominance. Rising FSH levels direct the multiplication of granulosa cells within the follicles and the expression of luteinizing hormone (LH) receptors in the granulosa cells. FSH then influences the activation of aromatase and p450 enzymes, which in turn cause the granulosa cells to discharge estrogen. Increasing levels of estrogen will trigger the production of gonadotropin-releasing hormone (GnRH). GnRH increases the production of LH. Androgen synthesis is induced by the presence of LH in the thecal cells, which ultimately sets the stage for ovulation. During this entire phase, increasing levels of estrogen in the blood will encourage the growth of the endometrium within the uterus and cause endometrial cells to create receptors for progesterone. These receptors help to prepare the endometrium for the presence of increasing levels of progesterone during the second half of the cycle. Hormonal levels must be consistently within normal ranges for ovulation and pregnancy to occur.
28 days is the average length of a menstrual cycle, but cycles can range anywhere from 21-35 days.