Heavy flow is most common in the teens and in perimenopause—both are times of the life cycle 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.

The good news is that, in a large study of pre- and perimenopausal women, heavy flow was not caused by endometrial cancer. This means that a diagnostic test for cancer called a D & C (a surgical procedure in which the endometrium is scraped off) is not necessary. Heavy flow was most common and occurred in 20% of women ages 40-44. In women ages 40 to 50, those with heavy flow commonly also have fibroids. However higher estrogen with lower progesterone levels causes both heavy bleeding and the growth of fibroids. Fibroids are benign tumors of fibrous and muscular tissue that grow in the muscle of the uterine wall; less than 10% come close to the endometrium and are called “submucus” fibroids. Only these rare fibroids could possibly influence flow. So fibroids are rarely the real cause for heavy flow and are not a reason to treat very heavy flow any differently.

Early in perimenopause when cycles are regular, approximately 25% of women will have at least one heavy period. Perimenopausal estrogen levels are higher and progesterone levels are lower. Progesterone levels are lower because ovulation is less consistent and short luteal phases (the portion of the normal menstrual cycle from ovulation until the day before the next flow) with fewer than 10 days of progesterone are common in perimenopause.

Some rare reasons for heavy flow are an inherited problem with bleeding (like hemophilia), infection or heavy bleeding from an early miscarriage.