Your fertility cycle is truly an amazing process that involves the secretion of various hormones at different points in the cycle to bring about ovulation and enable fertilization and implantation of the egg. If these events are successful pregnancy is achieved; if not, the process is repeated during the next cycle.
During menstruation, the endometrium (uterine lining) that had built up during the previous cycle is shed. The bloody tissue of the endometrium provides an excellent environment for the implantation of a fertilized egg, however, the hormone progesterone is necessary to maintain the thickness of the endometrium. When implantation doesn't take place, the levels of progesterone drop. As a result, the endometrium can no longer be maintained and it is shed in the process of menstruation.
Low progesterone levels also permit the pituitary gland to begin secreting another important hormone -- follicle-stimulating hormone (FSH). As the name implies, FSH stimulates the ovary to begin selection of another egg for ovulation during the new cycle. Actually, 15-20 eggs begin to grow towards maturity with the presence of FSH. Each egg is encased in its own follicle, which is a fluid-filled sac in the ovary. By approximately the 8th day of your cycle FSH causes one of the many follicles to become "dominant" and to fully develop the egg inside. The remaining eggs then disintegrate.
During the development of this new dominant follicle, estrogen is secreted. As the estrogen level rapidly increases, it causes your body to build up the endometrium again in anticipation of possible implantation by a fertilized ovum during the new cycle.
At this time, increased estrogen levels also cause glands around your cervix to secrete cervical fluid. Cervical fluid has several purposes in human reproduction: 1) it is a natural lubricant; 2) it provides nutrients for sperm to live; and 3) it provides sperm a means to travel to the fallopian tubes. Cervical fluid is what enables conception to take place -- without it, sperm would die within a few hours and would not be able to reach the ovum in the Fallopian tube. Early in your fertile phase, cervical fluid will be sticky and opaque, but as ovulation approaches, it normally becomes clear, stretchy, and even watery, like raw eggwhite. When you have eggwhite-quality cervical fluid, you are at the most fertile time of your cycle.
When estrogen levels reach their highest, they cause the pituitary gland to secrete high levels of luteinizing hormone (LH). It is this hormone which cause the dominant ovarian follicle to release the ovum in the process of ovulation. Ovulation normally occurs within 12-36 hours of this surge of LH.
For about two weeks after ovulation, progesterone is being released by what used to be the ovarian follicle, now called the corpus luteum, while it is attached to the ovarian wall. Progesterone causes the endometrium to remain conducive to the implantation of a fertilized ovum. It also causes your temperature to rise perceptibly, typically 0.4º Fahrenheit / 0.2º Celsius. Additionally, progesterone causes estrogen levels to drop, resulting in the drying up of your cervical fluid. This period of your cycle is known as the 'luteal phase' and, when followed by menstruation, its length is usually fairly consistent, normally lasting between 12 and 16 days, though it rarely varies by more than a day for any individual woman.
When fertilization occurs, it normally takes place in the Fallopian tube and life begins. The fertilized egg then travels to the uterus where it implants into the thickened endometrium. The first thing the fertilized ovum does after implantation is to secrete hCG (human chorionic gonadotropin), which in turn causes the corpus luteum to continue secreting progesterone. The progesterone helps maintain the endometrium in its thickened state and prevents menstruation and miscarriage.
If fertilization is not successful, progesterone levels drop, menstruation begins, and the cycle repeats itself.