The menstrual cycle begins on the first day of the period and often lasts three to six days. A new egg cell starts to mature in the ovaries when the period is over, and the mucous membrane or lining of the womb thickens in order to receive a fertilised egg.
Ovulation occurs twelve to fourteen days before the next period. A mature egg leaves the ovaries and moves into the Fallopian tube where it can be fertilised by a sperm cell. At the same time, the lining of the womb thickens in order to receive and nourish the fertilised egg. If the egg is not fertilised, the lining will push it out with bleeding. This takes place about two weeks after ovulation, and then the menstrual cycle starts all over again. The menstrual cycle lasts on average 28 days, but varies from woman to woman.
A girl might have to wait several months after her first period till the next one comes, and it can take one to two years before the period becomes regular.