What causes fibroids to grow?

Estrogen: Fibroids grow when exposed to naturally occuring estrogen. Since estrogen levels "swing" to higher highs and lower lows as a woman gets into her mid 30's and 40's, fibroids tend to grow and cause symptoms at these ages.

Unknown: No one knows for sure what causes a fibroid to permanently increase in size. In theory a temporary increase in size can be caused by increased blood flow during phases of the patients menstrual cycle.