The exact cause of endometriosis is unknown, however, there are several common theories as to why the condition may occur. Some of these causes are below, however, this is not an exhaustive list of all potential explanations on the origins of the endometriosis.
Retrograde menstruation
During menstruation, the blood that accompanies the shedding of the uterine lining (endometrium) exits the body through the vaginal cavity and vagina. Some theorists believe that a portion of the menstrual blood flow could travel backwards into the fallopian tubes and pelvic cavity instead of out of the body. This backwards flow (or retrograde menstruation) may carry endometrial cells within it, which can then implant and grow within the fallopian tubes and the surface of other pelvic organs. This theory is not completely understood however, as up to 70-90% of women have some degree of retrograde menstruation, but not all of these women develop endometriosis.1-6
Genetic factors
An individual's risk of developing endometriosis increases if they have one (or more) first-degree relatives with the condition. It has also been found that sets of twins often show patterns of endometriosis as well. These patterns of inheritance suggest that endometriosis may have a genetic component to its development.1-5
Hormone irregularities
The progression of endometriosis, as well as its onset and decline, seems to have a connection to hormones, specifically estrogen. For example, the onset of endometriosis generally does not happen before puberty, and symptoms tend to subside after menopause, both times of significant hormonal change. In addition, endometriosis often responds positively to hormonal therapies. This suggests that endometriosis may be, in part, hormone-dependent, and those who develop the condition may have a hormonal imbalance or issues with their hormone regulation. Another theory also suggests that hormones like estrogen can affect embryonic cells. These cells, which are in their earliest developmental stage, may be transformed into endometrial cells that will implant and grow into lesions during puberty.1,3-6
Transformation of peritoneal cells
This theory, also called the induction theory, suggests that peritoneum cells (the cells in the lining of the pelvic cavity and the pelvic organs) can transform into endometrial cells. These newly transformed endometrial cells can then build up and break down cyclically, just like the endometrial cells in the uterus. The process by which these cells transform is not well understood, however, it is thought to potentially be related to hormones or immune system-related factors.1-3,6
Surgery
When a surgery is performed on an area around the uterus or on the uterus, such as a cesarean section (C-section) or a hysterectomy, it is possible that some endometrial cells could be transplanted to the surgical site or surgical incision. For example, if a woman has a hysterectomy and endometrial cells are transplanted or attach to the surgical incision site during the operation, the woman may develop endometriosis at that site.2,4,6
Endometrial cell transport
It has been hypothesized that endometrial cells may be able to travel through the blood stream or lymphatic system to other, sometimes distant, parts of the body. When this happens, endometrial cells travel within these fluids and implant on other organs, structures, or protective linings in the body and continue to grow and break down as they normally would inside the uterus.2,5,6
Immune system disorder
Normally, the body's immune system recognizes when there are foreign invaders in a specific part of the body, or when certain cells are out of place. When these issues happen, the immune system works to fix the issue, typically by destroying the out of place cells. Since the body generally doesn't recognize that endometrial cells are growing and breaking down in the wrong location when a woman has endometriosis, it's possible that there is an issue with the immune system and its ability to detect problems. This theory is also supported by the fact that certain autoimmune conditions, including asthma, multiple sclerosis, and lupus, can accompany endometriosis. These co-occurring conditions suggest that an immune system issue may be present.1,3,4,6