Also known as dysmenorrhea, (Greek for "painful/bad menstruation"), menstrual cramps occur because of strong contractions of the uterus. Some women never have cramps; others may have cramping so severe as to incapacitate them for up to three days every cycle. Most women find that the amount of cramping they experience varies from cycle to cycle and changes over the years.

What causes menstrual cramps?

Fertile women generally release one ovum every month at ovulation; the endometrium, the inner lining of the uterus, becomes engorged with blood into which the ovum would implant if fertilization occured. If it does not, the endometrium is shed as menses, and the cycle begins anew. The uterus contracts to help expel the uterine lining, and particularly strong contractions cause the painful symptoms known as menstrual cramps.

There may be a hormonal aspect to menstrual cramps as well: some scientists believe that imbalances between estrogen and progesterone cause cramping, and studies have shown that women whose uterine linings produce high levels of prostaglandins have more severe cramping than women who don't. These unfortunates often also experience nauseau, vomitting, and headaches during their periods.

Women with a retroverted uterus (tipped or tilted uterus) may have more severe cramping than those without. Women on the Pill do not ovulate and so do not build up such a heavy layer of tissue in the uterus, resulting in lighter, artificial periods with less cramping. Women who have IUDs generally experience severe cramping, probably because the uterus is trying to expel the foreign body during menstruation.

In addition, certain pathologies of the reproductive system can cause what is known as "secondary dysmenorrhea". Typical causes include uterine fibroids, endometriosis, pelvic inflammatory disease, and adenomyosis. Secondary dysmenorrhea is much less common than "normal" or primary dysmenorrhea.

What can I do to relieve my cramps?

Just as each woman has her own experience of cramping, so she will find her own way of dealing with the cramps - except for secondary dysmenorrhea, which requires discovering the pathology underlying the condition and dealing with it, usually with drugs or surgery.

Non-medical therapies
Many women find that applying a heating pad or hot water bottle to the abdomen or lower back provides relief; a warm bath can help as well. Though your inclination might be to lie on the sofa and feel sorry for yourself, aerobic exercise actually helps to relieve cramps, if you can manage it; even a walk around the block or a few crunches encourages the body to produce those feel-good endorphins. Taking adequate amounts of calcium (1000 mg. per day) often helps to relieve cramping and PMS, and some women find zinc and B vitamins helpful as well. Try to relax: do some yoga, have a massage, drink a cup of mint or chamomile tea. Or have an orgasm; many women swear by this method!

Medications
In the over-the-counter category are acetomenophin (Tylenol) and non-steroidal anti-inflammatory drugs (NSAIDs: ibuprofen, naproxen sodium, or ketoprofen); NSAIDs are particularly effective at blocking the production of prostaglandins. Be aware that these medications can upset the stomach and are not suitable for those with ulcers or asthma. If you want to try these drugs, you may have to experiment to see which works best for you, and note that many women report the drugs work best if taken a day or two before menstruation starts.

In the "alternative medicine" category are a host of herbal drugs, including dong quai, cramp bark, motherwort, and St. John's wort. You might want to consult a naturopath before taking these.

Prescription medications for relieving menstrual cramps include stronger painkillers and hormonal contraceptives such as the Pill and Depo-Provera. Hormonal contraceptives have the added advantage of functioning as birth control, but are not suitable for smokers, who face an increased risk of blood clotting.

Preventive measures
Many health practitioners recommend general good-health practices such as exercising three times a week; and eating a good diet, which means cutting down or eliminating caffeine, sugar, (no chocolate!) salt, red meat, tobacco, and alcohol, particularly before menstruation. Cutting down on stress may help as well.

http://www.mckinley.uiuc.edu/health-info/womenhlt/mencramp.html
http://www.pcrm.org/research/menstrual.html
http://www.premenstrualsyndrome.com/cramps.html
http://www.medicinenet.com/Menstrual_Cramps/article.htm
http://www.mothernature.com/Library/bookshelf/Books/15/66.cfm