What causes very heavy menstrual bleeding?
This is not clear. Heavy flow is most common in the teens and in perimenopause—both are times of the lifecycle when estrogen levels tend to be higher and progesterone levels to be lower. Progesterone is made by the ovaries after ovulation. However, even though you may be having regular periods, it doesn’t mean you are ovulating! The lining of the uterus or endometrium sheds during a period. Estrogen’s job is to makes the endometrium thicker (and more likely to shed) and progesterone makes it thinner. Therefore it is likely that heavy flow is caused by too much estrogen and too little progesterone. However, this has not been well shown.