Like many processes in the body, fertility and reproductive processes are controlled by the body’s hormonal levels. As children undergo puberty, the changes they experience are caused by hormonal fluctuations. In girls, these hormonal fluctuations are comprised primarily by increases in the amount of estrogen, progesterone, follicle stimulating hormone (FSH) and luteinizeing hormone (LH) released in the body. Each of these hormones influences a component of reproductive activity in the body, and they work in concert to create a cycle, usually between 28 and 35 days in length, during which the lining of the uterus sloughs off and grows again in preparation for pregnancy.
The cycle begins with the first day of bleeding, when FSH is released by the pituitary gland and begins stimulating the "ripening" of eggs in the ovaries. Estrogen is then released from the ovaries, making the cervix (the opening to the uterus at the top of the vagina) and cervical mucous more amenable to penetration by sperm.
When estrogen levels in the body are high enough, the pituitary gland releases LH, which causes the "ripest" egg to be released from the ovary and travel to the fallopian tube (the tube that runs between the ovaries and the uterus) to await fertilization. After the egg is released, the follicle that burst to release it, now called the corpus luteum, continues to secrete a lesser amount of estrogen, which acts to return the cervix to its infertile state.
The corpus luteum also releases increased amounts of progesterone, which causes the lining of the uterus, or endometrium, to thicken in preparation for the implantation of the fertilized egg in the uterine wall, as well as stopping the ovaries from producing eggs and returning the cervical mucous to normal.
If fertilization of the egg does not occur, the corpus luteum dies and stops producing hormones. The drop in progesterone during this time can be a factor in premenstrual symptoms, such as depression and "bad moods" associated with PMS. The drop in hormones also causes the uterine lining to break down and be expelled by the body along with some blood, at which point the cycle begins again.
During the menstrual period, women may experience cramping as the uterus contracts somewhat to rid itself of the unused lining, and in the few days before the period, hormone changes in the body may contribute to water weight gain, bloating, and changes in mood. These symptoms are usually mild enough to be controlled with over the counter remedies, such as acetaminophen (Tylenol) ibuprofen (Advil or Motrin), or naproxen (Aleve), as well as combination medicines specially designed for PMS and menstrual symptoms, such as Midol, which contains both pain killers and an ingredient that helps reduce bloating.
In some cases, women may experience disruptive levels of depression, cramping, or heavy bleeding in association with their menstrual cycles. Women who suspect that their menstrual cycles are abnormal should consult a doctor, as any abnormalities in this cycle may be indications of an underlying problem, such as a hormonal imbalance, a disorder of the reproductive tract, or lack of adequate nutrition. However, most disruptive symptoms associated with menstruation can be resolved with the help of a doctor or medical care practitioner and a healthy lifestyle.