The uterus is the female reproductive organ that holds the fetus during pregnancy. When you’re not pregnant, your uterus is about the size and shape of an upside-down pear. The inner walls of the uterus are lined by the endometrium, a layer of tissue, which nourishes the fetus during pregnancy. In a woman who is not pregnant, this lining is shed each month as her menstrual flow. The walls of the uterus are made up of smooth muscle. The contraction of this muscle tissue allows a woman to give birth.

Fibroids are non-cancerous tumours, which develop on or in this muscle layer. The medical name for fibroids is uterine leiomyomas. A fibroid can be as small as an apple seed or as large as a melon.

Doctors talk about the size of a uterus enlarged by fibroids the same way they talk about the size of a pregnant uterus. So when a fibroid stretches the uterus to a 12- to 14-week size, the uterus is about the same size it would be if you were 12 to 14 weeks pregnant. This refers to a fibroid about the size of a small melon, or to several smaller fibroids.

Types of Fibroids

Fibroids are described according to their location.

Subserosal fibroids grow on the outermost layer of the uterus.

Intramural fibroids grow inside within the wall of the uterus.

Submucosal fibroids grow on the inner wall of the uterus. Because submucosal fibroids grow in the layer of muscle closest to the endometrium, they are the most likely to cause heavy menstrual bleeding.