If, while passing through the fallopian tube the ovum is fertilized by a sperm, implantation in the uterus occurs and the thickened lining helps support the pregnancy. When the ovum is not fertilized, this tissue and blood are shed. The proliferation of the uterine wall then begins once more in expectation of the next release of an ovum, and if conception does not take place, it sloughs off again. The process continues monthly until pregnancy occurs or until ovulation ceases at menopause. Menstruation is controlled by the hypothalamus and the pituitary gland and hormones, such as estrogen , which prepares the lining of the uterus, and progesterone , which helps maintain a pregnancy.
Many women experience painful menstruation, or dysmenorrhea.
The uterine contractions that result in the cramps experienced by these women appear to be caused by hormones called prostaglandins that are produced in the second half of the cycle. Oral contraceptives and other drugs that reduce the production of prostaglandins are sometimes used in treatment. The Columbia Encyclopedia, Sixth Edition.