It's natures way of preparing the body to secure the egg during the fertilization process. Conversely, if the time isn't right, the uterus begins a process of cleaning out the area so that the process can start all over again next month. Most women have relatively regular periods. The length of a normal menstrual cycle varies anywhere from three to five weeks, with menstruation lasting between three to seven days. Let's take a look at the actual physiology that takes place during your cycle.
Menstruation is controlled by a series of hormonal interactions between the brain and the ovaries.
FSH triggers the growth of follicles in the ovary where the egg is stored. As the follicle grows, it secretes estrogen in increasing amounts. This is called the proliferatory phase of the menstrual cycle. When the egg approaches maturity inside the follicle, the follicle releases a burst of progesterone and estrogen. Under the influence of LH, the follicle changes its function and begins to secrete less estrogen and more progesterone.