Retrograde menstruation theory

Sampson’s theory of retrograde menstruation is by far the most popular theory of the development of endometriosis and it probably explains the vast majority of cases. According to this theory endometriosis develops when menstrual fluid from the uterus flows backwards through the fallopian tubes and out into the pelvic cavity during the menstrual period. This process of backward flow is known as retrograde menstruation which is a normal process that occurs in the majority of women. When the menstrual fluid flows out of the ends of the fallopian tubes it is deposited onto the surrounding organs and tissues. The menstrual fluid contains blood and fragments of endometrium. Some of these fragments of endometrium are still living and they implant themselves on the surface of the tissue and begin to grow and function. These patches of implanted endometrium are known as endometrial implants, deposits or cysts. Although it is known that most women have retrograde menstruation, it is not known why only a small percentage actually develops endometriosis.

Blood and lymph transportation theory

Sampson’s theory of retrograde menstruation probably explains most of the cases of endometriosis where the implants are located in the pelvic cavity. However, it does not explain some of the rare cases where the implants are located in sites that are some distance from the pelvic cavity – for example, when the endometrial implants are located in the lungs, thighs or arms. In these circumstances it would appear that living fragments of endometrium somehow enter the veins or the lymph vessels and are transported around the body to another location where they implant in the surrounding tissue.

Accidental transplantation theory

The third way that endometriosis can develop is by accidental transplantation. In this situation, the endometrial fragments are transferred from the uterus to the new site during gynaecological surgery or the delivery of a baby. The fragments of endometrium become lodged in the muscle or tissue at the site of the surgical cut – such as in a caesarian or an episiotomy – where they implant. This theory explains the occurrence of endometriosis in surgical and episiotomy scars. Fortunately, this is not common.

Metaplastic theory

The last theory is known as the metaplastic theory and it is the least accepted of the four described here. According to this theory, women may have dormant cells in their pelvic cavities which have the ability to be transformed into endometrial cells in certain circumstances, such as when there is repeated irritation of the lining of the pelvic cavity. This irritation may be due to the presence of menstrual blood, infections or the hormones in the body.

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