What causes uterine fibroids, and why do they (sometimes) cause unusually heavy bleeding?
— Kristin Estabrook from Boston
Uterine fibroids, known medically as leiomyomas, are benign growths of uterine muscle. Genetics, race, hormones and previous pregnancies can all have a role in creating fibroids. There are several ways that these benign tumors can contribute to heavy menstrual periods.
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Fibroids are benign tumors of the muscle of the uterus, also called the myometrium. They are very common — by 50 years of age 70 percent of white women and 80 percent of black women will have at least one fibroid. For many of these women, fibroids are very small and/or cause no symptoms. They are often found incidentally — most commonly on an ultrasound, CT scan or M.R.I. that was performed for symptoms unrelated to fibroids.
For other women, fibroids can cause heavy or prolonged menstrual periods. Other symptoms may include:
Irregular bleeding — meaning bleeding not during the menstrual cycle.
Pelvic pressure and urinary incontinence, especially when fibroids are larger.
Complications in pregnancy such as miscarriage, preterm labor and even obstruction of labor, necessitating a cesarean section.
Fibroids are hormonally responsive. They are influenced by the reproductive hormones estrogen and progesterone and as such are not seen before puberty, typically shrink with menopause and can grow rapidly during pregnancy
How fibroids cause abnormal uterine bleeding is not completely understood. Fibroids that protrude into the uterine cavity and distort the lining of the uterus are most likely to cause abnormal menstrual bleeding, but the size and number of fibroids does not appear to be a factor. The current belief is that fibroid tissue affects the production of growth factors in the uterus, altering blood flow and how blood vessels respond during menstruation. There may be other mechanisms as well that are still undiscovered.
What Can Increase Your Risk?
Risk factors for fibroids include early puberty, probably caused by an earlier exposure to reproductive hormones. Several studies have linked obesity with fibroids, possibly because of the increased levels of estrogen created by adipose tissue. There is also a genetic component to fibroid risk as women with a family history of fibroids are more likely to have fibroids themselves.
There is a higher incidence of fibroids in black women who also tend to have more symptoms, especially at a younger age. The reasons for this are not fully known, but one theory is related to elevated levels of the enzyme aromatase in the fibroid tissue of black women; this enzyme is essential in the production of estrogen.
The greater the number of deliveries a woman has, the lower the risk of fibroids, but it is unclear how pregnancy protects against fibroids. Women who smoke cigarettes have a lower incidence of fibroids, perhaps because of the anti-estrogenic effect of smoking. (But this is not a reason to take up smoking!)