When a woman menstruates, the lining of the uterus is being shed and this is why there is bleeding. The reason that this happens is because she did not become pregnant during the menstrual cycle. In other words, a fertilized egg was not implanted in the uterus.
Certain hormones that are responsible for keeping women healthy are regulated during the menstrual cycle as well. An example is estrogen, which helps bones to be strong and healthy.
The cycle starts on day 1 of the first period and starts over again on day 1 of the next period. An average menstrual cycle lasts for 28 days, although this varies.