What's the best option to treat big (18 weeks) uterine fibroids?

Fibroids. Removal (myomectomy) if wanting children/more children. You may need to be on Lupron first to stop any bleeding and help shrink the fibroid a bit before removal.
Hysterectomy. If you are done with childbearing, removing the uterus may be the best course. It is likely that there are multiple fibroids, not just one big one.