Preface: At any age, I think women should understand their menstrual cycles. I assumed the reader has some knowledge of the human body and I think you do. ~
From the first occurrence of menstruation, known as “menarche”, to menopause the pituitary gland maintains a rhythm of follicle stimulating hormone (FSH) and luteinizing hormone (LH) secretion to produce, in most women, a regular menstrual cycle.
FSH stimulates the maturation of several immature eggs inside the ovary of which only one reaches maturity. These follicle cells (Graffian follicles) produce estrogen to build up the lining of the uterus also known as the endometrium.
In mid-cycle a surge of LH causes ovulation. The corpus luteum then forms and secretes progesterone, which further prepares the lining of the uterus for implantation of a fertilized egg. If indeed an egg becomes fertilized. (The corpus luteum is a hormone-secreting structure that develops in an ovary after an egg has been discharged but degenerates after a few days unless pregnancy has begun.)
If fertilization occurs the embryo produces human chorionic gonadotrophin (HCG) to maintain the stimulus to the corpus luteum and continued production of progesterone. It takes about a week for the fertilized egg to pass down the Fallopian tube and implant itself into the endometrium.
If fertilization does not occur the corpus luteum degenerates, progesterone production stops and the endometrium is shed because the uterus no longer needs the extra blood and tissue causing menstruation for about five days. This change in hormonal balance may be responsible for premenstrual tension and PMS.