It seems that the female hormone estrogen causes increased growth of fibroids. Sometimes fibroids worsen during pregnancy, when a woman's body produces more estrogen, or in menopausal women using hormone-replacement therapy. In menopausal women who choose not to take hormones, existing fibroids usually shrink because her body is producing less estrogen.
New fibroids are unlikely.
Fibroids can bulge toward either the inside or the outside of the uterus, and there's a vast range in size. Frequently fibroids occur in clumps, and they tend to enlarge and distort the uterus. The majority of women with fibroids don't have any symptoms. The fibroids are usually discovered during a pelvic exam. The fibroids stretch the lining, making it thinner and interfering with normal functioning.