A

MENSTRUAL CYCLE

The menstrual cycle is a periodic flow of blood and cells from the lining of the uterus in human females and the females of most other primates, occurring about every twenty-eight days. The beginning of menstruation, or menarche (the first menstrual period), typically starts between the ages of ten and seventeen and is a sign of readiness for childbearing.

During each cycle, the lining, or endometrium, of the uterus experiences a rapid generation of cells and vein-filled channels in preparation for pregnancy. Halfway through the cycle, an ovum (egg) is released from an ovary. The ovum passes through the fallopian tube, and if fertilized by a sperm, the ovum is implanted in the uterus, and the thickened lining helps support the pregnancy. If the ovum is not fertilized, the tissue and blood are shed.

The many myths and taboos related to menstruation have caused some cultures to chastise it as "un-clean" or a "curse." For a young girl, menarche is simply related to growth and body weight. Signs of puberty can begin after the age of eight, but early physical maturation may result in social pressure because of increased attention.