A fibroid starts as a single muscle cell in the uterus. For reasons that are not known, this cell changes into a fibroid tumor cell and starts to grow and multiply. Heredity may be a factor. It is thought that a muscle cell in the uterus may be "programmed" from birth to develop into a fibroid sometime-perhaps many years-after puberty (the start of menstrual periods).
After puberty, the ovaries produce more hormones, especially estrogen. Higher levels of these hormones may help fibroids to grow, although exactly how this might happen is not understood.