The menstrual cycle is a hormonal and physiological cycle mediated by hormones in the female body. Its purpose is reproduction---each month, the menstrual cycle ensures that a woman's body becomes prepared for implantation of a fertilized egg, should conception occur. There are several individual effects of the cycle, however; some are physical (and directly related to reproduction), and others are side effects of changing hormones.
Bleeding
Perhaps the most obvious effect of the menstrual cycle is the monthly menstrual period. Early in the menstrual cycle, rising concentrations of the hormones estrogen and progesterone cause the lining of the uterus to proliferate and thicken. In their book "What to Expect When You're Expecting," Heidi Murkoff and Sharon Mazel note that the purpose of this thick uterine lining is to provide a place for a fertilized egg to implant in the event that fertilization has occurred. If fertilization does not take place, however, hormone levels start to fall about two weeks after ovulation. The falling hormone levels trigger the lining of the uterus to slough off. As it does so, it is eliminated via the cervix, resulting in the vaginal bleeding that characterizes a menstrual period.
Ovulation
Some women can feel themselves ovulate, though to most, this major effect of the menstrual cycle passes largely unnoticed. Approximately two weeks after the last menstrual flow began, the hormones LH and FSH (both of which are produced in and released by a part of the brain called the anterior pituitary gland) spike in concentration. FSH causes an egg in one of the ovaries to ripen, while LH causes the release of that ripe egg into the fallopian tube. If sperm are present in the woman's body, they will fertilize the ripe egg. In the absence of conception, however, the egg simply passes unnoticed out of the woman's body together with her next menstrual flow.
Premenstrual Syndrome
Even though the classic symptoms of premenstrual syndrome, or PMS, are not strictly necessary to fertility and reproduction, they're side effects of changing hormone levels and are largely unavoidable as effects of the menstrual cycle. The Mayo Clinic notes that PMS includes mood changes, headache, cramps, and fatigue for many women. Breast tenderness is also quite common. In general, symptoms of PMS arise approximately a week prior to the menstrual period (about a week after ovulation), and most women find some relief from the symptoms once the period actually starts.
References
- "What to Expect When You're Expecting"; Heidi Murkoff and Sharon Mazel; 2008
- May Clinic: Premenstrual Syndrome
