It is likely that there are multiple triggers that cause a pluripotential cell or stem cell to develop into a leiomyoma cell instead of a normal muscle cell. That trigger may be lack of oxygen (hypoxia) or some other event. Once the leiomyoma cell is formed, it is independent of the normal regularly mechanisms that control cell growth. They grow and grow, creating a tumor.
Each fibroid tumor arises from a single genetic error or mutation. One fibroid does not directly cause another to appear. Different fibroids grow at different rates and at different times. They seem to behave independently, but similarly.
Some fibroid will tend to cause heavy bleeding, likely by affecting the uterus’ ability to contract effectively during menstruation.
They may also secrete bioactive chemicals that affect the growth of blood vessels in the uterine lining. Others will tend to grow rapidly and become very large, apply pressure to surrounding organs and creating pressure symptoms. Still other will grow to a certain size and then top growing.
Fibroids do not grow continuously and at a constant rate. Instead, fibroids actually grow in spurts. Then the grow very little or cease to grow for a period of time, then begin to grow again.
Fibroids also vary in shape and of course, size. Fibroids initially are spherical or nearly spherical when very small, but may become eliptical or even pancake-like in shape. Interestingly, fibroids closer to the outer surface are much flatter than those deeper in the wall. This is significant as physicians must be aware of the irregularity in shape in order to completely ablate the tumor.