Menorrhagia means heavy vaginal bleeding. In many cases, the bleeding has a known cause, like uterine fibroids. But in other cases the cause remains unknown. There's a medical threshold for menorrhagia -- losing more than 80 mL of blood in each menstrual cycle -- but most doctors now define menorrhagia by how much it affects your daily life: causing pain, mood swings, and disruptions in your work, sexual activity, and other activities.