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The principal modes are growth within the uterine wall, towards the cavity, and towards the outer surface: intramural uterine fibroids, submucosal uterine fibroids and subserosal uterine fibroids. “Fibrous tumour” is the historical label; the usual modern lesion is a leiomyoma, a benign growth of smooth muscle with a variable connective-tissue component, rather than a mass of collagen alone.^[ref]

An intramural uterine fibroid enlarges within the myometrium. It can remain enclosed by uterine muscle, push the cavity aside, or expand both inner and outer contours. Several nodules can give an irregularly enlarged uterus. Expansion compresses surrounding tissue into a cleavage plane or pseudocapsule; this differs from the destructive infiltrative growth typical of a malignant lesion.

A submucosal uterine fibroid grows towards the endometrium, projecting into and distorting the uterine cavity. It may remain broad-based or acquire a stalk as the surrounding wall is displaced. A pedunculated uterine fibroid can be pushed through the cervical canal and appear in the vagina as a uterine polyp. A polyp formed in this way is a mode of presentation of the same smooth-muscle tumour, not necessarily a different histological species. Its relation to the cavity helps explain why a comparatively small lesion may be associated with substantial bleeding.

A subserosal uterine fibroid projects outwards beneath the serosal covering. It may have a broad attachment or a long pedicle and become a large extrauterine-looking mass. Lateral outward growth can extend between the layers of the broad ligament; cervical growth can displace neighbouring pelvic structures. Rarely, a detached pedunculated lesion gains a blood supply from adjacent tissues and persists as a parasitic growth; this is not ordinary distant metastasis.^[ref]

The three directions are not mutually exclusive: a large nodule may affect both the uterine cavity and its external surface. Modern location classifications describe its relation to endometrium, myometrium and serosa more precisely, but those later type numbers are unnecessary to explain the anatomical modes requested here.

Growth is also distinct from secondary change. A poorly supplied area can undergo hyaline or cystic degeneration, haemorrhagic change or calcification; these changes alter consistency without establishing a new malignant mode of growth. Uterine fibroids often respond to the reproductive hormonal environment and commonly shrink after menopause, but neither every lesion's course nor its rate of growth is identical.^[ref] Location and attachment describe the mode of expansion; tissue examination establishes the nature of the tumour.