What Causes Fibroids In The Uterus?

I am 13 weeks pregnant and today at my checkup the my doctor told me there is a healthy pregnancy but I have fibroids in my uterus. These are the findings of an OBS ultrasound: Along the anterior and posterior portion of the uterus, there are small hypoechoic lesions, in keeping with intramural fibroids. The left and right ovary are normal in appearance. There is no free fluid nor adnexal mass indentified. The largest of the fibroids measures 3.1 x 3.6 cms along the mid inferior anterior uterine body. Solitary intrauterine pregnancy measuring 11 weeks, 0 days with cardiac activity identified. What causes these fibroids, can they turn to cancer and what should be done about such findings?