Uterine fibroids are benign (noncancerous) tumors of muscle tissue in the uterus. Fibroids occur when a single muscle cell in the wall of the uterus multiplies and grows to form a noncancerous tumor. Fibroids can change the shape or size of the uterus and sometimes the cervix (lower part of the uterus). Fibroids may affect fertility by blocking the uterus and fallopian tubes, which can make it difficult to conceive or may put any subsequent pregnancy at risk for miscarriage or early labor. Fibroids are found in 20% of women of reproductive age but are more common in African-American women (50%-80%).