Besides preparing the egg for fertilization, estrogen and progesterone stimulate the lining of the uterus. During the first two weeks following menstruation, estrogen causes the uterine lining to gradually rebuild itself. The inner mucous layer of glands of the endometrium begin to grow long, and the lining thickens through an increase in the number of blood vessels as well as the production of a mesh of fibers that interconnect throughout the lining. By midcycle, the lining of the uterus has increased three times in thickness and has a greatly increased blood supply.

After midcycle, usually around day 14, ovulation occurs; the egg is picked up by the fallopian tube and continues on to the uterus. The follicle that has produced the egg for that month (graafian follicle) is further stimulated after midcycle by LH and changes into a yellow body, or corpus luteum. It is the corpus luteum that secretes progesterone. Progesterone has further effects on the uterine lining. It causes a coiling of the blood vessels of the lining, which becomes swollen and tortuous and secretes a thick mucous.

If the egg is fertilized, it will implant on the uterine wall and the corpus luteum will continue to secrete progesterone. If no fertilization occurs, the corpus luteum begins to deteriorate and the progesterone levels decrease. The lining of the uterus starts to break down and menstruation begins.

Types and Causes of Menstrual Cramps
There are two types of menstrual cramps: primary dysmenorrhea, in which the pain itself is the main problem; and secondary dysmenorrhea, in which the pain is a consequence of another underlying health problem.

By far the most women suffer from the primary type of dysmenorrhea. This classification breaks down into two subtypes: primary spasmodic or congestive. Primary spasmodic dysmenorrhea is the type most commonly found in young women in their early teens to late twenties. It is more common in women who have never borne children. In fact, childbearing seems to mark the end of the primary spasmodic type of cramps in many women. It is characterized by sharp, viselike pains that are caused by a constriction and tightening of the uterine muscle. Some women also feel these sharp pains in the inner thighs and low abdominal muscles, and some additionally experience feelings of hot and cold, faintness to the point of passing out, nausea, vomiting, and bowel changes varying from constipation to diarrhea. The immediate cause of the cramping is that the uterine muscle and the blood vessels that supply the uterus are tight and contracted. Blood circulation and oxygenation to this area are diminished, so the metabolism of the uterus and pelvic muscles is decreased. Waste products of metabolism, such as carbon dioxide and lactic acid, build up, intensifying the pain and discomfort.

Primary Spasmodic Dysmenorrhea
Primary spasmodic dysmenorrhea has been linked to imbalances in the intricate hormonal system that operates throughout the menstrual cycle. First, medical researchers observed that women who don't ovulate, and consequently undergo only the estrogenic effects on the lining of the uterus, do not experience cramps. Therefore, progesterone needs to be present for menstrual cramps to occur. When cramps occur, the changes seen in the lining of the uterus are typical of those occurring during an ovulatory cycle when progesterone is present. Pain-free menses without ovulation are typically seen in women at both the beginning and end of their reproductive years, that is, in young teenagers who have just started to menstruate and in women who are transitioning into menopause. There is no evidence, however, that women with cramps actually have low levels of estrogen, or conversely, high levels of progesterone. It may be the interplay between the two hormones that influences the tension and constriction in the uter-ine muscle and blood vessels.