When a woman get sex within some days of the egg being released and sperm is present in her fallopian tube, the egg may grow to be fertilized (she becomes pregnant). But, if the egg is not fertilized her estrogen and progesterone levels will fall and the coating of the uterus (endometrium) will start breaking down - marking the start of her period.
The period consists of a small amount of blood and the endometrium. The bleeding is based on breaking of fine blood vessels within the womb as the lining detaches itself.
Every menstrual period generally remain about five days. Bleeding is more plentiful during the first two days.