The menstrual cycle, which occurs monthly basis in women of reproductive age, involves both the reproductive organs and the endocrine system. The overt physical changes that occur through the cycle---proliferation of the lining of the uterus, for instance, followed by shedding of that lining---are controlled by the release of several different hormones. The levels of these hormones in a woman's blood produce not only the obvious manifestations of menstruation, such as cramps and bleeding, but a host of additional symptoms and effects.
Follicle-Stimulating Hormone
According to "Human Physiology," a chemical called follicle-stimulating hormone (FSH) is produced in a portion of the brain called the anterior pituitary gland. Its primary role in women is to stimulate the growth of immature eggs, called follicles, in the ovaries. While several follicles begin to develop each month, generally a single follicle becomes larger than the rest and the smaller follicles atrophy. Only the largest follicle continues to develop under the influence of FSH. FSH levels increase steadily in the days just prior to ovulation and then decrease after ovulation occurs.
Estrogen
As follicles develop, they secrete the hormone estrogen, which causes the lining of the uterus to thicken with additional tissue and blood vessels in anticipation of the implantation of a fertilized egg. Estrogen also proves responsible for changes in cervical mucus that will eventually help sperm to reach a fertilized egg. The cervix is the narrow opening of the uterus into the vaginal canal and while it is plugged with thick, nearly impenetrable mucus during non-fertile periods, estrogen changes the nature of the mucus so that it provides not only a passage, but also something of an anatomical ladder for sperm. Estrogen levels gradually increase during development of a follicle, peaking around the time of ovulation. According to the Mayo Clinic, some women find that the lower levels of estrogen they experience around the time of menses can lead to headaches; higher estrogen levels, on the other hand causes fewer headaches.
Luteinizing Hormone
Like FSH, luteinizing hormone (LH) is produced in the anterior pituitary gland. Also like FSH, it becomes in highest concentration in a woman's body at the time of ovulation---in fact, LH proves directly responsible for the rupturing of a mature follicle, the event that releases an egg from the ovary. A mature follicle consists of two major parts: the egg contained within a follicle, which goes on to travel toward the uterus and the cells of the follicular wall, which remain with the ovary and form a group of hormone-secreting cells called the "corpus luteum" after ovulation.
Progesterone
Following ovulation, the corpus luteum produces large amounts of the hormone progesterone. The purpose of progesterone is to maintain the lining of the uterus. Because progesterone is produced by the corpus luteum, levels in the body become quite low before ovulation. Following ovulation, progesterone levels gradually increase, reaching a maximum approximately one week after ovulation. At this time, in the absence of fertilization of an egg, the corpus luteum will begin to atrophy and progesterone levels will fall. Falling progesterone causes the lining of the uterus to atrophy and slough, with blood and some tissue from the lining expelled as menstrual flow.
References
- "Human Physiology"; Lauralee Sherwood, Ph.D.; 2004
- MayoClinic.com: Estrogen and Headaches
