If you are like most women, you only have a vague idea of what your body does each month (or cycle) and you probably didn't care until the day you decided to become pregnant. Here is a quick glimpse of what menses (or your menstrual cycle) is and why.
Your period is not just vaginal bleeding; your uterus spent a lot of time preparing a thick lining for a baby. When pregnancy does not occur, your body sheds the lining and prepares to rebuild it for the next cycle. So what your period really is is a hormonal reaction triggered when you don't get pregnant. (That is why pregnant women don't have periods; the uterus will not shed the lining if it is being used to protect a fertilized egg.)
Note: The average cycle is 28 days, but can differ widely for each woman. Usually (and I use that term loosely) a woman's cycle will not differ from one cycle to the next. It won't be 25 days one cycle, then 40 days the next. For ease of understanding, we are going to stick to the 28-day example.
For the first half of your cycle (days 1-14), levels of the hormone estrogen are building up, signaling your body to rebuild the lining in your uterus in case conception occurs. At the same time, your ovaries are getting an egg (or two) ready to be released. About halfway through your cycle (day 14), this egg is released from the ovary, down the fallopian tube, and into the uterus. This is called ovulation.
This is when your egg is ready for fertilization. It just sits in your uterus, awaiting sperm to come and fertilize it. This is the optimum time for conception (more on that later; for now, let's just focus on what your body is doing at this time).
If the egg is not fertilized within a day or two, it will break apart and be reabsorbed by the woman's body. This disintegration will trigger a drop in hormone levels, signaling your body to shed that old lining. This is when your next period occurs. Then the whole process starts all over again.