It started on the egg cells of a woman. Once the egg leaves the ovary, it will travel together with the egg through the fallopian tubes towards the uterus. The ovary produces large volumes of hormones (estrogen and progesterone – which help to prepare the lining of the uterus for possible pregnancy). If pregnancy doesn’t happen, levels of estrogen and progesterone drop – the lining breaks down, and the blood, nutrients, and tissue flow out of your body causing your period. This cycle will start again.