menstrual cycle and menstruation

the cyclical nature of the reproductive life of a sexually mature female. One OVUM develops and matures within a Graafian FOLLICLE in one of the ovaries (see OVARY ). When mature, the follicle ruptures to release the egg, which passes down the FALLOPIAN TUBE to the UTERUS . The ruptured follicle becomes a temporary ENDOCRINE GLAND , called the CORPUS LUTEUM which secretes the hormone PROGESTERONE . Under the influence of progesterone, the uterus wall ( ENDOMETRIUM ) thickens and its blood supply increases in readiness for the implantation of a fertilized egg. If the egg is not fertilized and there is no PREGNANCY , the thickened endometrium is shed along with a flow of blood through the VAGINA (menstruation). The usual age at which menstruation starts is 12 to 15 but it can be as early as 10 or as late as 20. The duration varies and can be anything from 2 to 8 days, the whole cycle usually occupying about 29 to 30 days.