(MEN-stroo-ul SY-kul)

The monthly cycle of hormonal changes from the beginning of one menstrual period to the beginning of the next. During the monthly menstrual cycle, the lining of the uterus thickens to prepare for pregnancy. If pregnancy does not occur, estrogen and progesterone hormone levels begin falling. Very low levels of estrogen and progesterone tell the body to begin menstruation (the monthly shedding of blood and tissue from the lining of the uterus). The menstrual cycle usually lasts about 28 days, but this can vary from person to person. It normally starts during puberty and ends at menopause.