Each uterine fibroid develops from a single muscle cell in which certain genes have been damaged or altered. The genetic changes lead to a more rapid cell division than usual in response to stimulation of hormones and growth factors. The cell division is also uncontrollable which results in a large number of abnormal muscle cells and the formation of a visible growth.
It is quite common for muscle cells from different parts of the uterus to develop these genetic changes over a period of time. This results in the forming of many fibroids on the same uterus.
The cause of genetic changes is currently unknown. It is clear that there is no fibroid gene that can be passed from mother to daughters in a direct genetic inheritance manner. There is also no association of fibroids with dietary habits or history of childbearing.
Although the muscle cells made up of fibroids are abnormal in their genes, they are responsive to oestrogen, the female sex hormone.
During the years that a woman is menstruating, oestrogen stimulation leads to the continual growth of fibroids. In general, a fibroid increases in size by 1 cm a year.
During pregnancy, fibroids are known to grow more rapidly than during the non-pregnant period.
At menopause as oestrogen secretion ceases, many fibroids shrink in size slowly in the post-menopausal years. However, fibroids will not disappear completely, even years after menopause.
Some other growth factors are known to influence the growth of fibroids. These growth factors are not changed by menopause. This explains why some fibroids fail to shrink or may even continue to grow despite menopause.
Uterine fibroids can be classified according to their size (Table 1) or by their location in the uterus (Table 2):
Table 1: Classification of fibroids by size
Table 2: Classification by location
It is very common for fibroids of different sizes and locations to be present on the same uterus.
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